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Issued  December  2,  IM6. 

HAWAII  AGRICULTURAL  EXPERIMENT  STATION, 

J.    M.  WESTGATE,  Agronomist  in  Charge. 


Bulletin   No.  41. 


PHOSPHATE  FERTILIZERS  FOR  HAWAIIAN 
SOILS,  AND  THEIR  AVAILABILITY. 


WM.  T.  McG^OE; 

Former  Chemist 


EOEfiE,_ 


UNDEB  THE  SUPEEVISION  OK 

STATES  RELATIONS  SERVICE, 
Office  of  Experiment  Stations, 

U.  S.  DEPARTMENT  OF  AGRICULTURE. 


WASHINGTON: 

(.oVKRNMENT  PRINTING  OFFICE. 

1910. 


r 


Issued  December  2,  1916. 

HAWAII  AGRICULTURAL  EXPERIMENT  STATION, 

J.   M.  WESTGATE,  Agronomist  in  Charge. 


Bulletin   No.  41. 


PHOSPHATE  FERTILIZERS  FOR  HAWAIIAN 
SOILS,  AND  THEIR  AVAILABILITY. 


BY 


WM.  T.  McGEORGE, 

Former  Chemist. 


UNDER  THE  SUPERVISION  OF 

STATES  RELATIONS  SERVICE, 

Office  of  Experiment  Stations, 

U.  8.  DEPARTMENT  OF  AGRICULTURE. 


WASHINGTON: 

GOVERNMENT   PRINTING  OFFICE. 

1910. 


HAWAII    AGRICULTURAL    EXPERIMENT    STATION,    HONOLULU. 

[Under  the  supervision  of  A.  C.  True,  Director  of  the  States  Eelations  Service,  United  States 
Department  of  Agriculture.] 

E.  W.  Allen,  Chief  of  Office  of  Experiment  Stations. 

Walter  H.  Evans,  Chief  of  Division  of  Insular  Stations,  Office  of  Experiment  Stations. 

STATION  STAFF. 

J.  M.  Westgate,  Agronomist  in  Charge. 

J.  Edgar  Biggins,  Horticulturist. 

M.  O.  Johnson,1  Chemist. 

F.  G.  Krauss,  Superintendent  of  Extension  Work. 

J.  B.  Thompson,  Assistant  Agronomist,  in  Charge  of  Glenwood  Substation. 

Alice  R.  Thompson,  Assistant  Chemist. 

V.  S.  Holt,  Assistant  Horticulturist. 

C.  A.  Sahr,  Assistant  Agronomist. 

A.  T.  Longley,  In  Charge  of  Cooperative  Marketing  Investigations. 

J.  W.  Love,  Executive  Clerk. 

1  Appointed  July  25, 1915,  to  succeed  Wm.  T.  McGeorge,  transferred  to  U.  S.  Department  of  Agricul- 
ture, Bureau  of  Chemistry,  July  8, 1915. 

(2) 


LETTER  OF  TRANSMITTAL. 


Honolulu,  Hawaii,  July  1,  1916. 
Sir:  I  have  the  honor  to  submit  herewith,  and  recommend  for  pub- 
lication as  Bulletin  No.  41  of  the  Hawaii  Agricultural  Experiment 
Station,  a  paper  entitled  "  Phosphate  Fertilizers  for  Hawaiian  Soils, 
and  Their  Availability,"  by  Wm.  T.  McGeorge,  former  chemist  of  the 
station.  The  use  of  phosphate  fertilizer  on  Hawaiian  soils  is  of  ex- 
treme economic  importance,  owing  to  the  unavailability  of  the  large 
quantities  naturally  occurring  in  these  soils.  In  the  present  paper 
several  points  of  scientific  and  practical  importance  are  brought  out 
regarding  (1)  the  influence  of  phosphate  fertilizers  on  plant  growth, 
(2)  the  availability  of  the  naturally  occurring  phosphates,  (3)  the 
solubility  of  those  phosphates  as  measured  by  various  solvents,  and 
(4)  the  chemical  combinations  in  which  phosphoric  acid  exists  in 
Hawaiian  soils.  It  appears  that  the  primary  factor  in  the  nonavail- 
ability of  phosphoric  acid  in  Hawaiian  soils  is  the  peculiar  physical 
condition  of  the  soil  and  that  phosphate  fertilizers  should  be  applied 
in  a  soluble  form  to  produce  the  best  results. 
Respectfully, 

J.  M.  Westgate, 

Agronomist  in  Charge. 
Dr.  A.  C.  True, 

Director  States  Relations  Service, 

U.  S.  Department  of  Agriculture,  Washington,  D.  O. 

Recommended  for  publication. 
A.  C.  True,  Director. 

Publication  authorized. 
D.  F.  Houston, 

Secretary  of  Agriculture. 
(3) 


CONTENTS. 


Page. 

Introduction 7 

Availability  of  different  phosphates  applied  to  Hawaiian  soils 8 

Soil  types  used 8 

Method 8 

Experiment  1 9 

Experiment  II 15 

Experiment  III ". 17 

Experiment  IV 21 

Sand  cultures 24 

Discussion 25 

Summary 29 

Solubility  of  different  phosphates  in  Hawaiian  soils 30 

Solubility  of  phosphoric  acid  in  potted  soil 30 

Solubility  of  phosphate  fertilizer  after  addition  to  the  soil 32 

Solubility  of  phosphate  naturally  occurring  in  Hawaiian  soils 34 

Summary 40 

Acknowledgment 41 

Appendix 42 

The  determination  of  phosphoric  acid  in  Hawaiian  soils 42 

(5) 


ILLUSTRATIONS. 


Page. 
Platb  I.  Fig.  1. — Comparative  availability  of  phosphoric  acid  in  three  types 
of  soil;  only  nitrogen  and  potash  added.     Fig.  2. — Influence  of 

sodium  phosphate,  Experiment  I,  Millet  1 12 

II.  Fig.  1. — Comparative  influence  of  phosphates,  Experiment  I,  Millet 

I.  Fig.  2. — Comparative  influence  of  phosphates,  Experiment  I, 
Millet  1 12 

III.  Fig.  1. — Comparative  influence  of  phosphates,  Experiment  II,  Mil- 
let I.    Fig.  2. — Comparative  influence  of  phosphates,  Experiment 

II,  Millet  1 16 

IT.  Fig.  1. — Comparative  influence  of  phosphates,  Experiment  II,  Mil- 
let I.  Fig.  2. — Influence  of  cowpeas  on  the  availability  of  phos- 
phate rock,  Experiment  II,  Millet  1 16 

(6) 


PHOSPHATE  FERTILIZERS  FOR  HAWAIIAN  SOILS, 
AND  THEIR  AVAILABILITY. 


INTRODUCTION. 

It  has  been  shown  in  previous  publications  of  this  station1  that 
the  local  soils  are  rich  in  phosphoric  acid,  but  that  it  is  securely 
locked  up  in  insoluble  and  unavailable  combinations  with  the  highly 
basic  (and  probably  also  with  the  silicic)  soil  constituents.  Iron  and 
aluminum,  which  occur  in  abundance,  and  lime,  which  occurs  in 
widely  varying  amounts,  are  known  to  be  active  in  this  fixation,  and 
it  is  highly  probable  that  titanium,  which  occurs  in  large  amounts 
in  Hawaiian  soils,  is  also  an  important  factor  in  the  fixation.  In 
view  of  these  peculiarities  of  Hawaiian  soils  and  also  of  the  further 
fact  that  the  results  of  experiments  made  elsewhere  with  phosphate 
fertilizers,  have  been  so  contradictory  that  it  is  impossible  to  apply 
them  to  Hawaiian  conditions,  it  was  deemed  important  and  desirable 
to  make  a  careful  study  of  the  solubility  of  the  phosphoric  acid 
naturally  occurring  in  the  soils  as  well  as  of  the  behavior  of  various 
phosphates  when  applied  to  them. 

Investigations  were  therefore  undertaken  to  determine  the  avail- 
ability of  the  phosphoric  acid  in  untreated  and  fertilized  typical 
Hawaiian  soils  as  well  as  the  manner  in  which  phosphates  are  locked 
up  in  the  soil.2  These  investigations  included  several  series  of  pot 
experiments.  One  series  involved  five  successive  crops  and  extended 
over  a  period  of  two  and  one-half  years.  In  another  three  suc- 
cessive crops  were  raised.  In  connection  with  these  experiments 
determinations  were  made  of  the  total  phosphoric  acid  in  the  soil, 
the  percentage  of  phosphoric  acid  in  the  soil  and  in  the  various  phos- 
phates soluble  in  different  solvents,  and  the  quantities  of  phos- 
phoric acid  absorbed  by  the  crops  grown.  The  principal  crops  used 
in  the  experiments  were  millet,  cowpeas,  and  buckwheat.  Such 
experiments  were  considered  necessary  in  order  to  be  able  to  recom- 
mend the  most  economical  form  of  phosphate  to  use  on  the  peculiar 
soils  of  Hawaii. 

i  Hawaii  Sta.  Buls.  35  (1914)  and  40  (1915). 

*  The  pot  experiments  carried  on  in  this  work  were  first  undertaken  in  cooperation  with  the  Basic  Slag 
Committee  of  the  Association  of  the  Official  Agricultural  Chemists,  but  have  been  continued  and  broad- 
ened to  obtain  information  in  regard  to  phosphate  fertilizers  for  Hawaiian  soils. 

(7) 


AVAILABILITY    OF    DIFFERENT    PHOSPHATES    APPLIED    TO 
HAWAIIAN  SOILS. 

SOIL   TYPES    USED. 

Three  types  of  soil,  one  of  which  was  thought  to  be  very  deficient 
in  phosphoric  acid  were  used.  The  following  table  shows  the  chem- 
ical composition  of  the  soils  as  determined  by  digestion  with  hydro- 
chloric acid  of  specific  gravity  1.115: 

Table  I. — Composition  of  soils  used  in  the  experiments. 


Soil 
No.  3. 


Moisture 

Volatile  matter 

Insoluble  matter 

Ferric  oxid  (Fe203) 

Alumina  ( A1203) 

Titanium  oxid  (Ti02) . . . 
Manganese  oxid  (M113O4) 

Lime(CaO) 

Magnesia  (MgO) 

Potash  (K20) 

Soda(Na20) 

Sulphur  trioxid  (S03) . . . 
Phosphoric  acid  (P205) . . 


Soil 

Soil 

No.  1. 

No.  2. 

Per  cent. 

Per  cent. 

7.65 

12.61 

8.42 

12.19 

38.49 

36.74 

16.63 

15.96 

12.85 

17.52 

2.00 

3.50 

.24 

.08 

1.84 

.18 

8.71 

.34 

.39 

.34 

1.36 

.28 

.08 

.44 

.57 

.28 

Per  cent. 

4.02 

15.33 

31.60 

21.28 

21.37 

3.60 

.20 

.50 

.64 

.35 

.35 

.32 

.29 


Soil  No.  1  represents  the  type  to  be  found  in  and  about  Honolulu. 
It  has  a  sandy  texture  and  is  derived  in  part  from  the  disintegration 
of  black  volcanic  ash.  It  is  used  for  truck  gardening,  rice,  and 
bananas. 

Soil  No.  2  is  the  highly  ferruginous  type  of  red  clay  so  abundant  in 
the  islands. 

Soil  No.  3  is  a  red  soil  very  similar  to  No.  2,  differing  in  that  No.  3 
has  a  better  texture,  that  is,  less  clay. 

METHOD. 

The  method  used  in  preparing  the  pots  was  similar  to  that  pro- 
posed by  the  Basic  Slag  Committee  of  the  Association  of  Official 
Agricultural  Chemists.  Tin  pots  were  used  instead  of  clay  pots  in 
order  to  eliminate  the  loss  of  fertilizer  through  efflorescence.  Each 
pot  contained  6  pounds  of  soil.  The  forms  of  phosphate  used  were: 
Double  superphosphate  (46.25  per  cent  P205),  acid  phosphate  (19.49 
per  cent  P205),  four  different  Thomas  phosphate  slags  (A,  18.38  per 
cent  P205;  B,  19.04  per  cent  P205;  C,  13.31  per  cent  P205;  D,  15.86 
per  cent  P205),  phosphate  rock  (29.4  per  cent  P205),  commercial 
sodium  phosphate  (20.87  per  cent  P205),  trimagnesium  phosphate 
(54.1  per  cent  P205),  tribasic  potassium  phosphate  (33.4  per  cent 
P205),  dibasic  potassium  phosphate  (40.8  per  cent  P205),  monobasic 
potassium  phosphate  (52.2  per  cent  P205),  reverted  phosphate  (18.56 
per  cent  P205),  bone  meal  (27.76  per  cent  P205),  ferrous  phosphate 
(37.8  per  cent  P205),  ferric  phosphate  (31.8  per  cent  P205),  aluminum 
phosphate  (58  per  cent  P205),  and  titanium  phosphate  (34.1  per  cent 


P205).  All  phosphates  were  ground  to  about  the  same  degree  of 
fineness.  Each  pot  received  an  application  of  nitrogen  as  sodium 
nitrate  and  blood,  potash  as  potassium  sulphate,  and  lime  as  calcium 
carbonate.  The  lime  was  added  to  counterbalance  any  influences 
which  the  basic  material  in  the  slag  might  exert,  and  was  added  in 
excess  of  the  lime  requirement  as  determined  by  the  Veitch  method. 

The  crops  used  included  Japanese  millet,  cowpeas,  buckwheat, 
radishes,  and  turnips. 

Fertilizer  applications  are  represented  in  the  table  as  follows : 

N  =  0.06  per  cent  nitrogen  from  blood  and  0.01  per  cent  from 
sodium  nitrate. 

N1J4  =  0.09  per  cent  nitrogen  from  blood  and  0.015  per  cent  from 
sodium  nitrate. 

K  =  0.10  per  cent  potash  (K20)  from  potassium  sulphate. 

11^  =  0.15  per  cent  potash  from  potassium  sulphate. 

Ca  =  0.10  per  cent  calcium  carbonate  plus  that  required  by  the 
Veitch  method. 

Cax  =  0.15  per  cent  calcium  carbonate  plus  that  required  by  the 
Veitch  method. 

P^  =  0.007  per  cent  phosphoric  acid  (P205)  from  the  phosphate 
indicated. 

Px,  PlM,  P2  =  0.014,  0.021,  and  0.028  per  cent  phosphoric  acid, 
respectively. 

L  =  legumes. 

In  one  series  of  the  experiments,  green  manure  in  the  form  of 
well-macerated  cowpea  vines  was  added  at  the  rate  of  2  ounces  per 
pot;  in  the  other  no  green  manure  was  added.  All  applications  were 
made  in  duplicate  two  weeks  before  seeding.  The  pots  were  watered 
daily. 

EXPERIMENT   I. 

In  the  first  experiment,  soil  No.  2  was  used,  a  heavy  clay  soil  very 
deficient  in  available  phosphoric  acid.  In  Table  II  are  given  the 
numbers  of  the  pots,  kind  and  amount  of  fertilizer  added,  weight 
of  crop  both  green  and  dry,  weight  of  heads,  and  plants  per  two 
pots.  The  first  crop,  Japanese  millet,  was  planted  July  31  and 
harvested  October  20.  The  soil  was  then  dried  out,  aerated,  returned 
to  the  pots,  and  planted  to  cowpeas  November  17.  This  crop  was 
cut  on  January  17,  weighed  (each  pot  separately),  and  returned  to 
the  respective  pots.  Without  any  further  addition  of  fertilizer  the 
pots  were  planted  to  buckwheat  February  6,  which  crop  was  har- 
vested March  27.  The  soil  was  again  dried  out,  well  aerated,  and 
again  planted  to  millet  May  18,  without  further  addition  of  fertilizer. 
This  crop  was  harvested  on  August  10.  The  soil  was  again  dried, 
aerated,  and  well  mixed  as  above,  and  after  a  full  application  of  ni- 
trogen and  potash,  but  no  phosphoric  acid,  was  again  planted  to  millet. 
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12 

In  considering  the  above  data,  it  is  necessary  to  allow  for  seasonal 
variations.'  That  is,  while  it  is  possible  to  obtain  a  good  growth  the 
whole  year  round,  some  months  are  more  favorable  for  growth  than 
others.  Hence  differences  in  the  various  crops  may  not  necessarily 
be  due  to  the  action  of  the  fertilizers  alone.  Nevertheless,  the 
influence  of  the  different  fertilizers  in  the  same  series  is  comparative. 

The  data  show  that  this  soil  was  very  deficient  in  phosphoric  acid, 
since  in  nearly  every  instance  an  increase  in  weight  of  the  millet  plant 
followed  the  addition  of  phosphate  (see  Pis.  I  and  II).  When  phos- 
phoric acid  was  added  at  the  rate  of  0.007  per  cent  P205  (I\/2),  the 
soluble  phosphates  were  the  most  effective.  Phosphate  rock  was  the 
least  effective,  while  reverted  phosphate,  bone  meal,  and  slag  had  an 
almost  negligible  influence.  When  phosphoric  acid  was  added  at 
the  rate  of  0.014  per  cent  P205  (PJ,  practically  the  same  relation 
applied.  By  comparing  the  pots  with  and  without  legumes,  where 
no  phosphate  was  added,  plant  growth  seems  to  have  been  benefited 
by  the  use  of  green  manure.  When  added  with  the  phosphates,  the 
green  manure  did  not  show  a  very  active  influence  except  in  the  case 
of  phosphate  rock,  when  a  better  yield  was  obtained  with  the  use  of 
legumes. 

While  an  increase  in  growth  of  cowpeas  was  brought  about  by  the 
use  of  phosphate  fertilizer,  there  was  no  great  regular  variation  due 
to  the  various  phosphates.  When  applied  at  the  rate  of  0.007  per 
cent  P205,  the  slags  had  the  greatest  influence.  Applied  at  the  rate 
of  0.014  per  cent  P205,  sodium  phosphate  gave  a  higher  yield  than 
slags.  The  influence  of  acid  phosphate,  superphosphate,  and  phos- 
phate rock  was  quite  marked.  The  highest  yield  in  this  series  was 
obtained  through  the  use  of  sodium  phosphate  at  the  rate  of  0.021 
per  cent  P205. 

With  buckwheat,  as  with  the  two  previous  crops,  a  marked  influ- 
ence was  exerted  upon  growth  by  all  forms  of  phosphate.  The  rela- 
tive effect  of  the  different  fertilizers  was  quite  similar,  that  is,  while 
the  soluble  fertilizers  were  apparently  the  most  effective,  the  differ- 
ence was  very  small.  However,  it  may  be  safely  said  that  0.014  per 
cent  P205  was  more  effective  than  0.007  per  cent,  while  0.021  per 
cent  and  0.028  per  cent  were  still  more  effective. 

The  most  important  fact  brought  out  in  the  second  series  of  tests 
with  millet  was  the  marked  increase  in  effectiveness  shown  by  the 
phosphate  rock  and  slags,  which  were  about  equal  to  the  soluble  phos- 
phates. In  all  cases  0.014  per  cent  P205  produced  a  higher  yield  than 
0.007  per  cent.  Attentionis  called  to  the  fact  that,  when  added  at  the 
rate  of  0.007  per  cent  P205,  the  phosphate  rock  was  the  most  effective. 
But  when  added  in  larger  quantities,  the  soluble  phosphates  still 
maintain  their  superiority  as  regards  effectiveness  to  such  an  extent 
that  sodium  phosphate  added  at  the  rate  of  0.021  per  cent  P205 


Bui.  41,  Hawaii  Agr.  Expt.  Station. 


Plate  I. 


^J  IS 


&x 


■m 


Fig.  1— Comparative  Availability  of  Phosphoric  Acid  in  Three  Types  of  Soil; 
Only  Nitrogen  and  Potash  Added. 

Left  to  right:  First  two  pots,  red-clay  soil  No.  2;  second  pair,  red  soil  No.  3;  third  pair,  sandy 

soil  No.  1. 


Fig.  2.— Influence  of  Sodium  Phosphate,  Experiment  I,  Millet  I. 

Left  to  right:  Check  pot*;  sodium  phosphate  at  the  rate  oi"  0.007,  0.014,  and  0.0J1  pel  cent  P20i 


Bui.  41,  Hawaii  Agr.  Expt.  Station. 


Fig.  1.— Comparative  Influence  of  Phosphates,  Experiment  I,  Millet  I. 

Left  to  right  paired  pots:  Check;  slag  at  rates  of  0.007  and  0.014  per  cent  P2O5;  acid  phosphate, 

0.014  per  cent  P205. 


Fig  2— Comparative  Influence  of  Phosphates,  Experiment  I,  Millet  I. 

Left  to  right  paired  pots:  Phosphate  rock  at  rates  of  0.007,  0.014,  and  0.028  per  cent  P205;  acid 
phosphate,  0.007  per  cent  P2O0. 


13 

produced  a  larger  yield  than  phosphate  rock  at  the  rate  of  0.028  per 
cent  P205.  This  indicates  the  more  immediately  effective  influence 
of  soluble  phosphates. 

In  view  of  the  depressed  growth  of  the  preceding  crop,  it  seemed 
imperative  to  make  a  second  application  of  nitrogen  and  potash  for 
the  third  millet  crop  in  the  same  quantities  as  in  the  original  formula. 
In  this  way  it  would  be  possible  to  measure  further  the  cumulative 
effects  of  the  phosphates  and  to  determine  if  the  decrease  in  growth 
was  due  to  the  exhaustion  of  available  phosphate.  The  result  of  this 
test  was  a  further  decrease  of  growth,  proving  the  removal  of  available 
phosphate.  The  effectiveness  of  phosphate  rock  was  lowered  con- 
siderably as  compared  with  the  previous  crop.  The  influence  of  slags 
at  this  stage  was  the  most  important  result  on  this  crop.  Other  than 
the  slags,  the  soluble  phosphates  were  more  effective  than  the  insol- 
uble ones. 

After  repeated  failures  to  obtain  a  normal  growth  of  turnips  in  the 
pots,  due  to  insect  attacks,  experiment  with  this  crop  was  abandoned. 
However,  almost  complete  data  for  one  series  were  obtained,  and  the 
results  were  very  much  in  accord  with  the  first  crop  of  the  millet 
series.  Phosphate  rock  proved  the  least  available,  the  other  phos- 
phates were  much  more  effective  and  very  similar  in  their  action.  All 
applications  had  a  markedly  beneficial  influence  upon  the  growth  of 
the  turnips,  and  the  plants  to  which  lime  was  added  were  much*  larger 
than  those  treated  with  lime  and  legumes.  These  pots  were  later 
planted  to  radishes,  but  through  the  ravages  of  insects  and  other 
causes  the  experiments  with  this  plant  were  also  abandoned. 

After  standing  for  about  nine  months  in  the  pots,  the  soil  was  well 
mixed  and  again  seeded,  this  time  to  millet,  in  order  to  determine  the 
relative  availability  of  the  phosphates  after  allowing  ;tjie|  soil.'  to  lie 
fallow.  The  results  were  rather  surprising  in  that  all  potjs  produced 
a  very  poor  growth,  but  the  pots  fertilized  with  phosphorijc  acid  were 
in  advance  of  the  checks.  When  added  at  the  rate  of  0.007  per  cent 
phosphoric  acid,  acid  phosphate  gave  the  highest  yield,  while  phos- 
phate rock  was  second.  When  applied  at  the  rate  of  0.014  per  cent, 
sodium  phosphate  was  best,  closely  followed  by  the  slags,  phosphate 
rock,  and  acid  phosphate  in  the  order  given.  Since  only  a  small  per- 
centage of  phosphoric  acid  could  have  been  removed  by  the  turnips, 
the  stunted  growth  indicated  the  inability  of  the  lime  in  the  quantities 
added  to  retain  the  phosphates  as  calcium  phosphate. 

The  plants  from  the  first  two  crops  of  millet  and  the  crop  of  buck- 
wheat were  analyzed,  partly  to  determine  the  amount  of  phosphoric 
acid  removed  from  the  soil,  and  also  to  determine  whether  any  relation 
existed  between  the  types  of  phosphate  used  and  the  phosphate  con- 
tent of  the  grain  or  straw. 


14 

As  a  result  it  may  be  said  that  the  three  crops  removed  from  the  soil 
an  amount  of  phosphate  equal  to  that  added.  There  were,  however, 
a  few  exceptions,  especially  where  phosphate  rock  was  used.  These 
results  offer  information  regarding  the  stunted  growth  of  the  last 
crop  of  millet  and  the  increased  effectiveness  of  phosphate  rock  as 
compared  with  the  soluble  phosphates  in  the  second  crop. 

The  analyses  furnish  very  little  information  regarding  the  variation 
in  the  phosphate  content  of  the  grain  and  straw.  The  percentage  of 
phosphate  in  the  grain  was  increased  by  the  addition  of  phosphate 
fertilizer,  regardless  of  its  availability  in  some  cases,  while  in  others 
the  reverse  was  true.  The  same  may  be  said  regarding  the  straw, 
there  being  no  regularity  in  the  analyses.  The  results  are  given  in 
Table  III: 

Table  III. — Phosphoric  acid  content  of  grain  and  straw  as  influenced  by  phosphate 

fertilizers. 


Pot 
Nos. 


1,2 

5.6 

9,10.... 
13,14... 
17,18... 
21,22... 
25,26... 
29,30... 
33,34... 
37,38... 
41,42... 
45,46... 
49,50... 
53,54... 
57,58... 
61,62... 
65,66... 
69,70... 
73,74... 
77,78... 
81,82... 
85,86... 
89,90... 
93,94... 
97,98... 
101,102. 
105,106. 
109,110. 
113,114. 
117,118. 
121,122. 
125,126 
129,130. 
133,134 
137, 138 
141,142 
145, 146 
149, 150 
153, 154. 
157, 158 
161,162. 
165, 166 
169, 170 
173,174 


Fertilizer  added. 


N-K-Ca-L 

N-K-Ca 

N-K-Ca-L 

N-K-Ca 

N-K-Ca^P  i/«"(slag  A) .' '.'.'.'.'.'.'.'.'.'.'.. 

N-K-Ca-P  i/2  (slag  A) 

N-K-Ca-L-P  1/2  (slag  B) 

N-K-Ca-P  1/2  (slag  B) 

N-K-Ca-L-P  1/2  (slag  C) 

N-K-Ca-P  1/2  (slag  C) 

N-K-Ca-L-P  1/2  (slag  D) 

N-K-Ca-P  1/2  (slag  D) 

N-K-Ca-L-P  1/2  (acid  phosphate) 

N-K-Ca-P  1/2  (acid  phosphate) 

N-K-Ca-L-P  1/2  (phosphate  rock) 

N-K-Ca-P  1/2  (phosphate  rock) 

N-K-Ca-L-P  1/2  (sodium  phosphate) . 

N-K-Ca-P  1/2  (sodium  phosphate) 

N-K-Ca-L-P  1  (slag  A) 

N-K-Ca-P  1  (slag  A) 

N-K-Ca-L-P  1  (slag  B) 

N-K-Ca-P  1  (slag  B) 

N-K-Ca-L-P  x  (slag  C) 

N-K-Ca-P  1  (slag  C) 

N-K-Ca-L-P  x  (slag  D) 

N-K-Ca-P  1  (slag  D ) 

N-K-Ca-L-P  1  (acid  phosphate) 

N-K-Ca-P  1  (acid  phosphate) 

N-K-Ca-L-P  1  (phosphate  rock) 

N-K-Ca-P  1  (phosphate  rock) 

N-K-Ca-L-P  1  (sodium  phosphate).. 

N-K-Ca-P  1  (sodium  phosphate) 

N-K-Cax-L-P  1  (sodium  phosphate). 


N-K-Caz-P  1  (sodium  phosphate). 

N  1 1/2-K  1/2-Ca-L-P  1  (sodium  phosphate  . . 

N  1 1/2-K  1 1/2-Ca-P  1  (sodium  phosphate) . . . 

N-K-Ca-L-P  1 1/2  (sodium  phosphate) 

N-K-Ca-P  1 1/2  (sodium  phosphate) 

N-K-Ca-L-P  2  (phosphate  rock) 

N-K-Ca-P  2  (phosphate  rock) 

N-K-Ca-L-P  1/2  (superphosphate) 

N-K-Ca-P  1/2  (superphosphate) 

N-K-Ca-L-P  1  (superphosphate) 

N-K-Ca-P  1  (superphosphate) 


Millet  I. 


Grain.    Straw 


Per  ct. 
0.293 
.399 
.250 
.425 
.454 
.592 
.392 
.598 
.395 
.619 
.333 
.464 
.350 
.376 


.584 
.300 
.278 
.229 
.200 
.578 
.367 
.252 
.265 
.307 
.443 
.584 
.358 
.345 
.666 
.471 
.329 
.537 
.189 
.377 
.277 
.421 
.510 


.351 


.388 
.369 


Per  ct. 
0.121 
.172 
.099 
.092 
.192 
.277 
.196 
.209 
.173 
.263 
.116 
.157 
.135 
.126 
.219 
.240 
.179 
.133 
.077 
.079 
.108 
.051 
.097 
.134 
.143 
.171 
.275 
.162 
.172 


209 
224 
336 
,153 
.209 
,128 
,195 
,260 
.216 


.123 
.221 
.135 


Buckwheat. 


Grain.    Straw 


Perct. 
0.383 
.404 
.413 
.433 
.483 
.638 
.503 
.515 
.453 
.478 
.488 
.408 
.462 
.616 
.472 
.562 
.443 
.509 
.593 
.645 
.518 
.598 


.581 
.428 
.428 
.617 
.675 
.498 
.507 
.494 
.732 
.509 
.519 
.538 
.680 
.657 
.752 
.650 
.764 
.652 
.576 
.682 
.522 


Perct. 
0.085 
.115 
.146 
.102 
.127 
.122 
.129 
.156 
.175 
.116 
.133 
.115 
.119 
.128 
.116 
.202 
.118 
.136 
.238 
.152 
.114 
.185 
.101 
.099 
.097 
.068 
.125 
.346 
.099 
.124 
.104 
.188 
.119 
.099 
.157 
.244 
.167 
.219 
.169 
.246 
.120 
.182 
.159 
.139 


Millet  II. 


Grain.    Straw 


Perct. 
0.218 
.180 
.337 
.208 
.368 
.237 
.356 
.274 
.365 
.361 
.350 
.325 
.236 
.249 
.283 
.299 
.368 
.406 
.349 
.488 
.422 
.407 
.397 
.316 
.262 
.294 
.502 
.448 
.314 
.507 
.521 
.383 
.464 
.421 
.387 
.452 
.547 
.672 
.672 


.349 
.627 


Perct. 
0.156 


,094 


,187 
,187 
.203 
,230 
,293 
,263 
.253 
.230 
.173 
.179 
.168 
.120 
.144 
.119 
.114 
.108 
.158 
.128 
.139 
.148 
.106 
.098 
.191 
.163 
.153 
.130 
.148 
.140 
.129 
.154 
.169 
.139 
.145 
.106 
.104 
.115 
.105 
.109 
.081 
.104 


15 


EXPERIMENT    II. 

Another  series  of  experiments  with  millet  was  made  with  this  same 
type  of  soil — namely,  No.  2 — the  results  of  which  are  given  in  Table  IV. 
Fertilizers  were  added  hi  the  same  amounts  as  in  the  previous  series, 
and  the  soil  was  otherwise  given  similar  treatment.  This  was  deemed 
necessary  to  verify  some  of  the  results  obtained  in  the  first  series. 
After  the  second  crop  of  millet,  a  complete  fertilizer  was  added  before 
the  planting  of  the  third  crop.     (See  Pis.  Ill  and  IV.) 

Table  IV. — Effect  of  fertilizers  on  weight  of  crops  in  Experiment  II 


Pot 
Nos. 


Fertilizer  added. 


Millet  I. 


Weight  of 
crop. 


Green.   Dry 


Num- 
ber of 
plants. 


Millet  II. 


Weight  of  crop. 


Green.   Dry.  Heads 


Num- 
ber of 
plants. 


Millet  III.i 


Weight  of 
crop. 


Green.   Dry 


Num- 
ber of 
plants. 


1,2 

5,6 

9,10.... 
13,14... 
17,18... 
21,22... 
25,26... 
29,30... 
33,34... 
37,38... 
41,42... 
45,46... 
49,50... 

53,54... 

57,58... 

61,62... 

65,66... 

69, 70. . . 

73,74... 
77,78... 
81,82... 
85,86... 
89,90... 
93,94... 
97,98... 
101,102. 
105, 106. 

109,110. 

113,114. 

117,118 

121,122. 

125, 126. 

129,130 

1.33,134. 

137, 138. 

141,142. 

14o;14'J. 

149, 150. 


N-K-Ca-L 

N-K-Ca 

N-K-Ca-L 

N-K-Ca 

N-K-Ca-L-P  i/2  (slag  A) 

N-K-Ca-Pi/2(slagA).. 

N-K-Ca-L-P  iMslag  B) 

N-K-Ca-P  1/2  (slag  B).. 

N-K-Ca-L-P  x/2  (slag  C) 

N-K-Ca-P  i!t  (slag  C)... 

N-K-Ca-L-P  i/2  (slag  D) 

N-K-Ca-P  x/,  (slag  D).. 

N-K-Ca-L-P  x/2  (acid 
phosphate) 

N-K-Ca-P  i/j  (acid 
phosphate) 

N-K-Ca-L-P  1/2  (phos- 
phate rock) 

X-K-Ca-P  1/2  (phos- 
phate rock) 

N-K-Ca-L-P  ilt  (sodi- 
um phosphate) 

N-K-ca-P  ilt  (sodium 
phosphate) 

N-K-ca-L-P^slagA). 

N-K-Ca-P  1  (slag  A) . . . 

N-K-Ca-L-P  1  (slag  B) 

N-K-Ca-P  1  (slag  B)... 

N-K-Ca-L-P  1  (slag  C). 

N-K-Ca-P  1  (slag  C).... 

N-K-Ca-L-P  1  (slag  D) 

N-K-Ca-P  1  (slag  D) . . . 

N-K-Ca-L-P  1  (acid 
phosphate) 

N-K-C  a-P  1  (acid  phos- 
phate)  

N-K-Ca-L-P  1  (phos- 
phate rock) 

N-K-Ca-P  1  (phosphate 
rock) 

N-K-Ca-L-P  1  (sodium 
phosphate) 

N-K-Ca-P  1  (sodium 
phosphate) 

N-K-Ca-L-P  1  (sodium 
phosphate) 

N-K-Ca-P  1  (sodium 
phosphate) 

Nn;.  K  u  2  Ca-L-P  , 
(sodium  phosphate) . . 

X  1  1  ,-K  ,  1/2-Ca-P  , 
(sodium  phosphate) . . 

N-K-Ca-L-P  i,/2 (sodi- 
um phosphate ) 

N-  K-C  a-P  1 1/2  (sodium 
phosphate) . . .' 


Gm. 

8.1 
(2) 

2.0 
.9 
23.4 
23.9 
28.5 
27.5 
20.5 
11.7 
23.3 
14.0 

33.7 

34.5 

18.7 

(2) 

54.0 

40.4 
32.7 
29.7 
30.7 
35.5 
33.7 
16.5 
24.2 
11.0 

39.0 

41.5 

17.0 


42.5 
65.0 
45.5 
61.0 
27.2 
37.0 
42.5 
48.2 


Gm. 
4.6 


.2 
15.5 
13.8 
18.4 
19.0 
12.8 

7.6 
15.0 

8.4 

24.8 
23.8 
12.3 


37.4 

25.9 
21.1 
20.6 
20.9 
24.7 
21.6 

9.7 
18.5 

7.0 

26.9 
28.3 
10.9 
5.8 
26.0 
43.3 
32.2 
39. 2 
17.7 
22.8  I 
31.4 
32.2  I 


Gm. 
51.5 
(2) 
53.0 
23.0 
63.6 
98.9 
89.0 
79.5 
101.0 
83.0 
64.0 
63.6 

55.5 

46.2 

100.0 

68.5 

95.0 

57.6 
79.0 

108.5 
98.0 

101.0 
85.6 

102.0 
80.0 
86.0 

95.5 

77.0 

97.0 

76.0 

108.0 

90.0 

90.5 

92.0 

86.5 

110.5 

116.0 

113.6 


Gm. 
24.6 


24.3 
10.0 
30.4 
45.2 
39.5 
41.6 
44.5 
38.2 
34.0 
30.0 

27.1 

23.9 

45.6 

30.5 

41.6 

29.3 
35.0 
48.0 
43.2 
46.5 
40.0 
45.1 
36.0 
40.5 

43.7 

35.0 

43.2 

36.0 

48.8 

41.5 

47.7 

45.0 

41.4 

51.6 

52.6 

50.7 


Gm. 
6.4 


Gm. 


Gm. 


6.3 
2.25 
7.2 
11.1 
10.3 
8.8 
9.3 
8.2 
8.4 
7.2 

5.6 

4.8 

9.6 

8.8 

6.1 

5.3 
4.0 
4.6 
7.7 
5.2 
7.8 
8.4 
8.1 
8.3 

6.5 

6.2 

8.3 

8.1 

6.0 


8.1 
7.0 
10.1 
11.0 
8.3 
9.3 


14.1 
14.8 
15.9 
66.4 
65.2 
94.1 
113.5 
83.5 
77.5 
13.5 
92.5 

84.1 

87.2 


27.5 
6       90.4 


85.7 

66.5 
103.7 
122.4 
111.7 

72.5 
125.5 

72.5 
117.0 


Complete  reapplication  of  fertilizer. 


2  No  crop 


122.2 

66.7 

30.5 

17.2 

130.  7 

68.5 

113.7 

Ho.  7 

71.0 

77. .", 

141.2 

157.2 


8.3 

8.0 

9.0 

32.1 

30.8 

49.2 

49.5 

42.5 

37.5 

7.5 

47.0 

41.5 

40.7 

5.5 

12.7 

43.0 

41.5 
29.0 
50.2 
43.5 
49.0 
30.5 
61.0 
35.5 
54.6 

52.0 

33.0 

15.5 

8.5 

55.5 

28.3 

49.4 

52.5 

25.8 

33.3 

59.3 

73.0 


16 


Table  IV. — Effect  of  fertilizers  on  weight  of  crops  in  Experiment  II— Continued. 


Pot 

Nos. 


153,154. 
157, 158. 
161,162. 
165,166. 
169, 170. 
173,174. 


Fertilizer  added. 


N-K-Ca-L-P  2  (phos- 
phate rock) 

N-K-Ca-P  2  (phosphate 
rock) 

N-K-La-L-P  1/2  (super- 
phosphate)   

N-K-Ca-P  1/2  (super- 
phosphate)   

N-K-Ca-L-Px  (super- 
phosphate)   

N-K-(  a-P  1  (super- 
phosphate)   


Millet  I. 


"Weight  of 
crop. 


Green.    Dry 


Gm. 
16.7 

0) 

40.7 

30.5 

46.5 

48.5 


Gm. 
11.7 


26.6 
21.5 
30.0 
32.7 


Num- 
ber of 
plants. 


Millet  II. 


Weight  of  crop. 


Green.    Dry.  Heads 


Gm. 
98.5 

87.5 

61.0 

44.5 

89.5 

79.5 


Gm. 
44.2 

39.1 

38.1 

22.7 

45.0 

36.8 


Gm. 
9.4 

8.7 

5.6 

3.6 

7.4 

6.3 


Num- 
ber of 
plants. 


Millet  III. 


Weight  of 
crop. 


Green.    Dry, 


Gm. 
34.5 

66.8 
131.9 

78.4 
149.6 
117.5 


Gm. 
17.0 

30.0 

63.8 

40.5 

70.2 

57.6 


Num- 
ber of 
plants. 


1  No  crop. 

The  first  crop  was  planted  February  2  and  harvested  May  2,  the 
second  crop  planted  May  8  and  harvested  August  10,  and  the  third 
crop  planted  October  12  and  harvested  January  6.  The  soil  was 
well  mixed,  aerated,  and  dried  in  the  air  for  a  short  time  between 
plantings. 

The  results  of  the  first  series  were  almost  completely  verified  by 
this  second  planting.  The  pots  to  which  no  phosphate  was  added 
produced  a  very  poor  growth.  All  applications  of  phosphate 
increased  the  growth,  the  soluble  phosphates  having  the  greatest 
influence,  while  phosphate  rock  had  the  least. 

The  second  crop  of  millet  in  this  series  again  verified  the  results  of 
the  first  series  with  marked  regularity.  When  added  at  the  rate  of 
0.007  per  cent  P205,  phosphate  rock  and  the  slags  were  most  efficient. 
The  availability  of  the  phosphate  rock  when  added  at  the  rate  of 
0.014  per  cent  P205,  was  very  high,  but  did  not  surpass  that  of  sodium 
phosphate,  and  the  latter,  when  added  at  the  rate  of  0.021  per  cent 
P205,  produced  a  heavier  growth  than  the  former  when  added  at  the 
rate  of  0.028  per  cent  P205.  This  apparently  is  further  evidence  of 
the  more  lasting  effect  of  phosphate  rock  when  added  in  small 
quantities,  and  of  the  superiority  of  the  soluble  phosphates  when 
added  in  larger  quantities. 

At  the  same  time  that  the  pots  in  Experiment  I  were  ready  for  the 
third  planting  of  millet  those  in  Experiment  II  were  also  ready  for 
the  third  crop  of  millet.  In  view  of  this  fact,  another  complete 
application  of  nitrogen  and  potash  was  made  in  the  former  with  the 
results  already  given,  while  a  second  application  of  phosphate  was 
made  to  the  latter  in  order  to  compare  the  two.  The  most  obvious 
results  were  the  rapidity  of  growth  in  Experiment  II  as  compared 
with  that  in  Experiment  I  and  the  increase  in  wTeight  of  plants  at 


Bui.  41,  Hawaii  Agr.  Expt.  Station. 


Plate  III. 


Fiq.  1.— Comparative  Influence  of  Phosphates,  Experiment  II,  Millet  I. 

Left  to  right:  Check;  slag,  phosphate  rock,  acid  phosphate,  superphosphate,  and  sodium  phos- 
phate, each  used  at  the  rate  of  0.014  per  cent  P205. 


Fig.  2.— Comparative  Influence  of  Phosphates,  Experiment  II,  Millet  I. 

Left  to  right  paired  pots:  Check,  phosphate  rock  at  rates  of  0.007,  0.014,  and  0.02s  per  cent  PjOft. 


Bui.  41,  Hawaii  Agr.  Expt.  Station. 


Plate  IV. 


Fig.  1.— Comparative  Influence  of  Phosphates,  Experiment  II,  Millet  I. 

Left  to  right  paired  pots:  Check;  sodium  phosphate  at  rates  of  0.007,  0.014,  and  0.021  per 

cent  P2O5. 


Fiq.  2—  Influence  of  Cowpeas  on  the  Availability  of  Phosphate  Rock, 
Experiment  II,  Millet  I. 

Left  to  right  paired  pots:  Phosphate  rock  at  the  rate  of  0.007  per  cent  P2O5  without  cowpeas; 
same  with  cowpeas;  phosphate  rock  0.014  per  cent  P2O5  without  cowpeas;  same  with 
cowpeas. 


17 

maturity.  Phosphate  rock  again,  as  in  the  original  application, 
proved  the  least  available.  The  other  phosphates  were  quite  similar 
in  their  action,  and  all  were  more  effective  than  phosphate  rock. 
Large  applications  of  the  latter  were  little  more  effective  than  small 
applications. 

EXPERIMENT    III. 

In  view  of  the  fact  that  the  soil  used  in  Experiments  I  and  II  was 
deficient  in  phosphoric  acid,  two  soils  less  deficient  in  this  constituent 
and  of  a  much  better  mechanical  texture  were  chosen  for  further  tests. 
In  soil  No.  I  the  phosphates  had  little  influence  upon  plant  growth, 
for  which  reason  only  one  planting  was  made.  In  soil  No.  3  there 
was  a  slight  effect,  and  two  crops  were  grown  in  this  series.  In  the 
red  clay  soil  (No.  2),  the  plants  did  not  stool,  but  in  soils  Nos.  1  and  3 
there  was  excessive  stooling,  and  for  this  reason  the  number  of  stools 
as  well  as  of  plants  is  indicated  in  the  tables. 

The  treatment  of  the  pots  was  somewhat  modified  as  follows: 
Only  one  slag  was  used  in  this  series,  and  additions  were  made  of 
trimagnesium  phosphate,  tripotassium  phosphate,  dipotassium  phos- 
phate, monopotaSsium  phosphate,  reverted  phosphate,  and  bone  meal. 

The  pots  of  soil  No.  1  were  seeded  June  18  and  harvested  September 
10.  Those  of  soil  No.  3  were  seeded  June  23  and  harvested  Septem- 
ber 18;  they  were  then  aerated,  mixed,  and  replanted  to  millet 
October  12,  and  harvested  January  11.  The  results  are  given  in 
Table  V. 

All  pots  of  soil  No.  1  produced  an  excellent  growth  of  millet  re- 
gardless of  fertilizer.  The  growth  in  the  pots  containing  sodium  phos- 
phate was  slightly  greater  than  in  the  other  pots,  but  this  soil  showed 
itself  to  be  very  high  in  available  phosphate. 

Soil  No.  3  proved  to  be  slightly  deficient  in  available  phosphoric 
acid,  and  an  increase  in  plant  growth  resulted  from  all  phosphate 
applications.  Phosphate  rock  was  again  the  least  effective  of  all  the 
phosphates.  Sodium  phosphate  and  superphosphate  produced  the 
largest  increase,  while  the  results  from  slags  and  acid  were  very 
good.  Reverted  phosphate  and  bone  meal  were  very  ineffective. 
The  monobasic,  dibasic,  and  tribasic  potassium  phosphates  were 
used  to  determine,  if  possible,  any  influence  due  to  basicity  of  the 
salt,  but  no  such  relation  was  apparent. 

The  second  millet  crop  on  soil  No.  3  gave  very  little  information  of 
additional  value.  In  this  crop  the  plants  did  not  stool,  and  partly 
for  this  reason  the  weight  of  the  plants  was  considerably  reduced; 
hence  the  decrease  in  plant  growth  noted  here  must  be  attributed 
primarily  to  seasonal  factors,  although  removal  of  readily  available 
phosphate  may  have  been  a  minor  factor.  Phosphate  rock  proved 
to  be  a  very  ineffective  form  of  phosphate,  and  the  soluble  phosphates 
gave  the  best  results. 
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EXPERIMENT   IV. 

Hawaiian  soils  contain  abnormally  large  quantities  of  iron  and 
aluminum.  Investigators  have  attributed  the  unavailability  of  phos- 
phoric acid  in  soils  directly  to  the  presence  of  these  two  elements, 
and  it  is  quite  generally  held  that  the  presence  of  adequate  amounts 
of  lime  will  prevent  phosphates  from  combining  with  iron  and  alumi- 
num. The  first  series  of  pots  in  Experiment  IV,  therefore,  was 
planned  to  determine  what  influence  the  lime  has  on  the  availability 
of  phosphoric  acid  in  the  soils.  This  series  of  pots  was  divided  into 
two  main  groups,  one  receiving  no  lime,  the  other  being  limed  at  the 
rate  of  0.10  per  cent  calcium  carbonate  in  excess  of  the  lime  require- 
ment as  determined  by  the  Veitch  method. 

The  same  three  soils  were  used  in  this  series  as  in  previous  experi- 
ments. Some  of  the  pots  were  green  manured  with  legumes,  as  indi- 
cated in  the  second  column.  The  data  are  presented  in  the  same 
manner  as  in  the  previous  tables.  Only  one  crop  of  millet  was 
grown  in  soil  No.  1,  but  the  tests  upon  soils  Nos.  2  and  3  were  repeated 
by  a  second  planting.     The  results  are  given  in  Table  VI. 

The  addition  of  lime  to  soil  No.  1  caused  only  a  slight  increase  in 
growth  of  the  plants.  Probably  this  was  due  to  the  high  percentages 
of  lime  and  magnesia  already  present  in  this  soil. 

In  soil  No.  3  a  large  increase  followed  the  application  of  lime  with 
the  soluble  phosphates,  but  not  its  application  with  phosphate  rock, 
This  indicates  that  lime  assists  the  plants  in  assimilating  phosphoric 
acid,  but  whether  this  is  a  chemical  or  physical  phenomenon  is  yet 
to  be  proved. 

In  soil  No.  2  the  effect  of  lime  was  much  more  striking.  Here  the 
growth  of  millet  was  increased  in  every  instance  where  lime  was  ap- 
plied with  the  phosphates.  This  soil  is  a  heavy  clay  type;  No.  3 
contains  much  less  clay,  and  No.  1  the  least  clay.  It  is  entirely  pos- 
sible that  there  is  a  relation  between  the  influence  of  the  lime  and  the 
amount  of  clay  present  in  the  soil.  The  most  striking  effect  of  the 
lime  in  case  of  soil  No.  2  was  upon  the  character  of  the  plant.  In  all 
pots  without  lime,  the  millet  came  up  in  clumps  like  grass  and  grew 
to  a  height  of  only  about  6  inches,  while  the  addition  of  lime  produced 
normal  plants. 

On  replanting  the  millet  in  soils  Nos.  2  and  3  the  results  were 
somewhat  different.  The  lime  had  apparently  lost  its  influence  in 
most  cases,  more  especially  in  soil  No.  3,  which  contained  less  clay. 

The  second  part  of  Experiment  IV  was  planned  to  determine  the 
availability  of  ferrous,  ferric,  and  aluminum  phosphates  as  com- 
pared with  the  other  phosphates.  Soils  Nos.  2  and  3  were  used  in 
this  work,  the  results  of  which  are  given  in  Table  VII. 


22 


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Table  VII. — Availability  of  iron  and  aluminum  phosphates. 


Soil  No.  2. 

Soil  No.  3. 

Fertilizer  added. 

"Weight  of  crop. 

Number 
of  plants. 

Weight  of  crop. 

Number 

Green. 

Dry. 

Green. 

Dry. 

ofplants. 

N-K-Ca-L-P  (ferrous  phosphate) 

Grams. 

7.0 

0)    • 

Grams. 
3.8 

1 

Grams. 
45.5 
41.0 
51.5 
55.0 
60.5 
88.5 
11.3 
70.0 
16.0 
29.5 
34.9 

Grams. 

.  21.5 
17.5 
35.5 
28.0 
31.8 
55.3 
5.3 
33.3 
9.5 
17.8 
20.0 

8 

N-K-Ca-P  (ferrous  phosphate > 

g 

N-K-L-P  (ferrous  phosphate) 

8 

42.5 
35.5 

18.6 
16.7 

8 
8 

8 
8 
8 

N-K-L-P  (ferric  phosphate) 

N-K-Ca-L-P  (aluminum  phosphate) 

N-K-Ca-P  (aluminum  phosphate) 

35.0 
35.5 

15.8 
16.7 

7 
7 

2 

8 

7 

N-K-Ca  (check) 

8.5 

5.0 

4 

8 

N-K-L  (check) 

8 

i  No  crop. 

In  soil  No.  2,  ferrous  phosphate  apparently  had  a  toxic  influence 
upon  the  millet,  while  ferric  and  aluminum  phosphate  proved  to  be 
very  available  types  of  fertilizer.  They  were  more  readily  available 
than  phosphate  rock,  about  equal  to  basic  slag,  but  less  readily  assimi- 
lated than  soluble  phosphate. 

In  case  of  soil  No.  3,  ferrous  phosphate  produced  a  good  growth, 
due  probably  in  part  to  the  fact  that  this  soil  is  more  open  and  the 
ferrous  salt  may  have  been  oxidized  to  ferric  phosphate.  Hence  it 
may  be  said  that  both  the  iron  salts  and  the  aluminum  salt  are  avail- 
able sources  of  phosphoric  acid,  more  so  in  this  type  of  soil  than  phos- 
phate rock,  about  the  same  as  reverted  phosphate  and  bone  meal, 
but  less  than  the  other  phosphates. 

In  view  of  the  action  of  lime  upon  the  availability  of  the  phosphates 
shown  in  the  previous  table  (Experiment  IV),  pots  were  prepared  to 
which  the  iron  and  aluminum  phosphates  were  applied  with  and 
without  lime.  The  results  showed  iron  phosphates  to  be  more  avail- 
able without  lime  in  this  type  of  soil.  The  opposite  relations  held  for 
aluminum  phosphate. 

SAND    CULTURES. 

A  further  test  of  the  phosphates  was  made  in  sand  culture  to  de- 
termine more  precisely  the  action  of  the  salts  wThen  not  under  the  in- 
fluence of  complex  soil  conditions.  Eighteen  pots  of  silica  sand  were 
prepared,  to  each  of  which  equal  weights  of  nitrogen  and  potash  fer- 
tilizer were  applied  in  addition  to  the  following,  which  *were  run  in 
duplicate:  Ferrous,  ferric,  aluminum,  sodium,  titanium,  and  acid 
phosphates,  phosphate  rock,  and  slag.  Two  check  pots  received 
nitrogen  and  potash  but  no  phosphoric  acid.  Each  phosphate  pot 
contained  the  same  weight  of  phosphoric  acid  (P205). 


-  25 

After  about  two  weeks  the  plants  were  growing  most  vigorously  in 
the  pots  containing  sodium  and  acid  phosphates.  Ferrous  phosphate 
was  very  toxic  at  this  stage,  but  the  plants  were  able  to  survive  and 
partly  overcome  the  toxicity.  After  two  months'  growth  phosphate 
rock  and  ferrous  phosphate  had  produced  the  smallest  plants.  Ferric 
and  aluminum  phosphate  had  produced  the  most  vigorous  plants, 
the  former  being  slightly  better  than  the  latter.  Sodium,  acid,  and 
titanium  phosphates,  and  slag  were  slightly  less  favorable  than 
ferric  and  aluminum  phosphate. 

DISCUSSION. 

The  availability  of  a  phosphate  will  vary  with  the  type  of  soil,  the 
climatic  conditions,  and  the  character  of  the  crop  to  which  it  is  applied. 
In  view  of  this,  due  consideration  must  be  given  to  the  fact  that  the 
preceding  data  have  been  obtained  with  only  three  types  of  soils  and 
a  limited  number  of  crops  grown  under  modified  conditions. 

The  type  of  Hawaiian  soil  on  which  phosphate  fertilization  is  most 
effective  is  that  known  as  the  red  ferruginous  clay.  According  to 
Hilgard,1  it  would  be  expected  that  when  water-soluble  phosphates 
are  applied  to  tins  type  of  soil,  the  phosphoric  acid  would  be  with- 
drawn from  useful  action.  Hence  any  excess  that  the  plant  is  not 
able  to  immediately  utilize  becomes  inert  and  useless,  that  is,  it  com- 
bines with  the  oxids  and  hydroxids  of  the  trivalent  metals,  iron  and 
aluminum,  and  in  this  form  it  is  for  all  practical  purposes  insoluble 
and  inaccessible  to  the  crop.  For  this  reason  he  advised  the  use  of 
diflicultly-soluble  phosphates,  such  as  bone  meal  and  basic  slag, 
which  react  less  readily  with  iron  and  aluminum.  Until  very  recent 
years  this  has  been  the  generally  accepted  theory  among  soil  chemists. 

The  results  of  more  recent  investigations,  however,  indicate  that 
iron  and  aluminum  phosphates  are  readily  available  to  plants,  in 
many  cases  more  so  than  the  insoluble  forms  of  calcium  phosphate, 
such  as  bone  meal,  slags,  and  floats.  Recent  work  at  the  Wisconsin 
Experiment  Station,2  for  example,  has  shown  that  9  out  of  10  plants 
tested  made  better  growth  when  fertilized  with  aluminum  phosphate 
than  with  calcium  phosphate,  while  6  of  the  10  made  better  growth 
with  ferric  phosphate. 

In  view,  therefore,  of  the  uncertainty  on  the  subject,  the  peculiar 
character  of  Hawaiian  soils,  and  the  practical  importance  of  the  matter, 
it  was  deemed  necessary  to  study  carefully  the  behavior  of  various 
phosphates  on  typical  Hawaiian  soils. 

The  phosphates  used  as  the  basis  of  the  preceding  experiment  arc 
all  of  commercial  importance.  Other  phosphates  were  added  to  the 
series  in  order  to  obtain  information  relative  to  the  availability  of 

i  Soils.    New  York  and  London,  1906,  p.  9S7.  »  V'Lsconsin  Sta.  Bui.  240  (1914),  p.  22. 


26 

the  former.  Basic  slag  meal  is  a  waste  product  in  the  manufacture 
of  steel,  and  in  addition  to  15-20  per  cent  phosphoric  acid,  also 
contains  about  30  per  cent  lime.  It  has  thus  far  found  little  appli- 
cation as  a  fertilizer  in  Hawaii.  Acid  phosphate  and  double 
superphosphate  ai»e  [manufactured  from  phosphate  rock.  The  acid 
phosphate  is  the  simpler  product  and  contains  about  20  per  cent 
phosphoric  acid  as  monocalcium  phosphate.  The  double  super- 
phosphate, through  a  modification  in  the  process  of  manufacture, 
contains  about  45  per  cent  phosphoric  acid  as  monocalcium  phos- 
phate. This  class  of  phosphate  fertilizers,  more  especially  the  former, 
is  widely  used  in  Hawaii.  It  may  be  used  with  advantage  on  practi- 
cally all  types  of  soil  which  show  a  deficiency  of  phosphoric  acid. 
Phosphate  rock,  known  as  floats  when  finely  ground,  is  the  natural 
tricalcium  phosphate  found  in  large  deposits  in  various  parts  of  the 
world,  but  it  is  little  used  in  Hawaii.  Bone  meal  is  quite  widely 
used  as  a  source  of  phosphoric  acid  in  Hawaii,  but  is  effective  only 
where  the  soil  receives  plenty  of  water.  It  is  especially  effective  if 
applied  in  connection  with  green  manure.  Reverted  phosphate, 
which  is  primarily  dicalcium  phosphate,  is  being  used  to  an  increas- 
ing extent  in  the  islands.  It  is  prepared  by  adding  lime  to  acid 
phosphate. 

The  results  of  the  experiments  indicate  that  the  soluble  phosphates 
are  the  most  effective  on  Hawaiian  soils,  especially  those  of  the  red- 
clay  type.  As  already  indicated,  this  is  contrary  to  the  conclusions 
reached  by  others  in  regard  to  the  application  of  soluble  phosphates 
to  soils  high  in  iron  and  aluminum  oxids. 

Iron  and  aluminum  phosphates  are  readily  available  sources  of 
phosphoric  acid  in  Hawaiian  soils,  the  former  more  so  in  the  first 
crop  in  the  absence  of  added  lime.  In  sand  cultures  the  trivalent 
phosphates  surpass  the  calcium  phosphates.  Hence,  since  the  pre- 
cipitated phosphates  of  iron,  aluminum,  and  even  titanium  are 
available  to  plants,  factors  other  than  chemical  combination  must 
be  considered  in  order  to  explain  the  apparent  insolubility  of  phos- 
phoric acid  in  Hawaiian  soils. 

That  the  phosphoric  acid  of  the  red-clay  soils  of  Hawaii  exists  in 
some  form  extremely  unavailable  to  plants  is  proved  by  the  data 
obtained  in  Experiments  I  and  II  (pp.  10,  15).  The  soluble  phos- 
phates were  the  most  effective  on  the  first  crop.  Their  effectiveness 
decreased  somewhat  in  the  following  crops.  The  phosphate  rock  was 
least  effective  at  the  outset,  and  its  effectiveness  increased  and 
then  decreased  as  compared  with  other  phosphates,  if  the  weight  of 
succeeding  crops  may  be  used  as  a  criterion.  Through  fermenta- 
tion changes  and  chemical  action  the  availability  of  the  phosphate 
rock  was  increased  to  such  an  extent  that  the  plants  of  the  second 
millet  crop  to  which  this  fertilizer  was  applied  had  a  larger  reserve 


27 

to  draw  from  and  made  a  better  growth  than  those  fertilized  with 
soluble  phosphates.  The  most  convincing  proof  of  this  statement  is 
furnished  by  comparing  the  results  of  P1/2  with  those  of  Pt,  Px  1/2,  and 
P2.  In  the  second  millet  crop  P1/2  phosphate  rock  gave  the  best 
growth,  P1/2  soluble  phosphate  being  comparatively  more  exhausted. 
By  comparing  Px  1/2  soluble  phosphate  with  P2  phosphate  rock  the 
former  is  found  to  have  been  considerably  more  effective.  Addi- 
tion of  nitrogen  and  potash  fertilizer  failed  to  prevent  the  decrease 
in  plant  growth  due  to  removal  of  available  phosphates,  but  upon  a 
further  addition  of  phosphoric  acid  a  marked  stimulation  resulted. 

It  has  been  claimed  that  lime  has  a  depressing  effect  upon  the 
assimilation  of  phosphoric  acid  from  phosphate  rock.  The  results 
obtained  in  sand  culture  did  not  support  this  view.  It  is  possible 
that  the  apparent  depressing  effect  of  phosphate  rock  in  the  red-clay 
soil  is  due  primarily  to  the  conditions  in  the  soil  unfavorable  to  the 
assimilation  of  the  insoluble  phosphates.  This  kind  of  phosphate 
fertilizer  is  rendered  soluble  and  available  as  plant  food  mainly  by 
the  action  of  carbon  dioxid  and  organic  acids  produced  by  the  micro- 
organisms of  the  soil.  Aeration  in  this  soil  is  very  low  and  carbon 
dioxid  is  present  in  almost  negligible  quantities.  Hence  if  the 
amount  of  carbon  dioxid  present  is  a  measure  of  bacterial  activity, 
which  is  known  to  vary  widely  in  soils  according  to  physical  and 
chemical  conditions,  then  the  primary  agent  upon  which  phosphate 
rock  depends  for  its  availability  is  lacking.  A  further  factor  may  be 
found  in  the  fact  that  the  acidity  of  this  type  is  not  due  to  the  presence 
of  organic  acids,  but  to  conditions  the  nature  of  which  is  yet  to  be 
determined. 

The  table  shows  the  marked  increase  in  assimilation  of  phosphate 
rock  in  the  green-manured  pots  as  compared  with  those  unmanured, 
indicating  that  the  fermentation  of  green  manure  and  loosening  of  the 
soil  assisted  the  millet  in  assimilating  the  phosphate  rock.  The  same 
factors  influence  the  availability  of  bone  meal,  which  is  greater  in 
humic  soils  where  acid-forming  bacteria  are  present. 

The  amount  of  phosphoric  acid  present  in  each  pot,  according  to 
the  absolute  analysis  of  the  soil,  was  approximately  18.0  grams,  while 
that  added  as  fertilizer  ranged  from  0.19  gram  (P^)  to  0.76  gram 
(P2).  The  analyses  of  the  plants  show  that  the  total  amount  of 
phosphoric  acid  removed  by  the  two  crops  of  millet  and  one  of  buck- 
wheat, in  most  instances  is  equal  to  that  added  at  the  rate  of  F12, 
in  several  instances  to  that  at  the  rate  of  P2,  and  in  one  case  to  that 
at  the  rate  of  Pn/2.  The  third  crop,  which  was  not  analyzed,  was 
the  most  stunted  in  growth  of  all  the  series,  indicating,  as  the  analyses 
show,  that  the  major  part  of  the  available  phosphoric  acid  had  been 
removed  by  preceding  crops,  leaving  only  that  naturally  occurring 
in   the  soil.     It  is   plainly  evident   that   the  millet  was  unable   to 


28 

utilize  this  phosphate,  although  it  was  present  in  amounts  23  times 
the  largest  application  and  90  times  the  smallest.  The  plants  were 
low  and  the  heads  very  small. 

In  Experiment  II,  in  which  another  application  of  phosphate  was 
made  after  harvesting  the  second  crop,  the  results  indicated  that  the 
effectiveness  was  to  a  slight  extent  proportional  to  the  amount  of 
phosphate  added.  The  lowest  yield  was  from  theP^  pots,  while  the 
highest  was  from  the  Px  tj2  pots.  The  results  as  a  whole  prove  the 
unavailable  condition  of  the  phosphate  naturally  occurring  in  the 
red-clay  soil,  and  show  further  that  the  addition  of  any  type  of 
phosphate  to  the  soil  either  assists  in  the  assimilation  of  that  already 
present,  or  itself  acts  as  a  source  of  phosphate  to  the  plants. 

The  fate  of  soluble  phosphates  when  added  to  the  red-clay  soils  and 
their  influences  upon  the  physical  condition  have  been  quite  thor- 
oughly dealt  with  in  previous  bulletins  of  this  station.1  The  fixing 
power  of  this  soil  for  phosphoric  acid  has  been  shown  to  be  so  very 
high  and  rapid  that  a  loss  of  phosphate  by  drainage,  through  over- 
application,  is  impossible.  The  fixation  of  the  calcium  phosphate  is 
greater  and  more  rapid  than  that  of  the  sodium  phosphate,  but  the 
sodium  of  the  latter  acts  as  a  strong  deflocculating  agent  and  would  be 
more  completely  distributed  throughout  the  soil.  This  probably 
explains  its  greater  effectiveness  as  compared  with  acid  phosphate, 
which  tends  to  flocculate  the  soil  particles. 

Lime  was  added  throughout  the  experiments  on  the  assumption 
that  it  would  cause  a  reversion  of  the  soluble  phosphate  and  thus 
delay  its  ultimate,  combination  with  the  trivalent  oxids.  From 
the  results  obtained,  the  conclusion  is  obvious  that  a  normal  applica- 
tion of  lime  is  not  capable  of  holding  the  phosphate  in  reserve  in  a 
form  available  for  the  plant  and,  furthermore,  that  the  benefit  de- 
rived from  the  application  of  the  lime,  while  it  may  be  due  in  part 
to  its  chemical  activities,  is  primarily  physical.  The  action  is  only 
temporary,  and  its  influence  is  exerted  to  the  greatest  extent  in  the 
first  crop.  The  nature  of  its  action  is  a  flocculation  of  the  clay 
particles  which  temporarily  disturb  the  colloidal  condition  in  which 
the  iron  and  aluminum  oxids  and  hydroxids  exist.  These  compounds, 
together  with  some  silica,  combine  to  form  the  clay  present  in  this 
type  of  soil.  The  most  important  function  of  this  flocculation  is  to 
hinder  or  perhaps  only  delay  the  occlusion  of  the  phosphoric  acid  by 
the  colloids.  That  the  soil  does  finally  return  to  such  a  state  in  due 
time  following  the  application  of  lime  is  indicated  by  the  physical 
condition  of  the  soil  in  the  pots  after  the  third  crop  of  millet  in 
Experiment  II,  and  the  further  fact  that  it  had  reached  a  state  of 
apparent  acidity,  as  determined  with  litmus  paper.     At  the  same 

i  Hawaii  Sta.  Buls.  35  (1914)  and  40  (1915). 


29 

time  there  was  a  decrease  in  availability  of  phosphate  as  measured  by 
the  growth  of  millet. 

In  the  types  of  Hawaiian  soils  that  contain,  abnormally  high  per- 
centages of  lime,  the  phosphate  is  always  present  in  an  available  form. 
This  indicates  that  the  lime,  when  present  in  such  large  quantities, 
is  able  to  perform  its  function  in  spite  of  the  excessive  quantities  of 
iron  and  aluminum,  but  in  order  that  it  may  exercise  this  function,  it 
must  be  present  in  amounts  far  in  excess  of  that  indicated  by  the 
determination  of  the  lime  requirement. 

The  conclusion  is  evident  that  the  unavailability  of  the  phos- 
phates in  the  clay  soils  of  Hawaii  is  not  due  entirely  to  their  chem- 
ical combination  with  iron  and  aluminum  as  phosphates  but  to 
other  causes  of  a  far  more  complex  nature.  The  experiments  re- 
ported in  this  bulletin,  as  well  as  those  of  several  other  investigators, 
have  shown  the  power  of  certain  plants  to  assimilate  the  precipitated 
phosphates  of  iron  and  aluminum  both  when  applied  in  sand  cul- 
tures and  when  applied  to  soils.  On  the  other  hand,  it  will  be 
shown  later  in  this  bulletin  that  the  major  part  of  the  phosphates  of 
Hawaiian  soils  does  exist  in  the  form  of  iron  and  aluminum  phos- 
phates and  that  the  addition  of  soluble  phosphates  results  in  a  rapid 
combination  with  these  elements.  The  answer  to  the  question  why 
plants  can  not  assimilate  the  phosphates  of  Hawaiian  soils  is  probably 
to  be  found  in  the  realm  of  soil  physics  as  indicated  above. 

SUMMARY. 

(1)  Hawaiian  soils  are  uniformly  higher  in  phosphate  than  main- 
land soils,  but  this  is  less  available,  especially  in  the  heavy  clay 
types. 

(2)  The  unavailability  of  the  phosphoric  acid  in  the  ferruginous-clay 
soils  is  not  due  entirely  to  chemical  combination  but  partly  to  physical 
occlusion . 

(3)  Phosphoric  acid  should  be  applied  to  this  type  of  soil  in  the  form 
of  soluble  phosphates  and  in  light  applications  at  frequent  intervals 
if  rapid  returns  are  anticipated. 

(4)  In  most  locations  it  is  poor  economy  to  add  bone  meal  or  other 
difficultly  soluble  phosphates  to  Hawaiian  soils  because  they  already 
contain  enough  insoluble  phosphate  to  grow  crops  for  an  indefinite 
number  of  years  provided  the  plants  had  the  power  to  assimilate  it. 

(5)  In  wet  districts  (uplands)  phosphate  rock,  bone  meal,  basic 
slag,  or  reverted  phosphate  should  be  very  effective,  more  especially 
so  if  applied  to  highly  organic  soils  or  used  in  systems  of  diversified 
agriculture  where  they  may  be  incorporated  with  green  manure  crops. 

(6)  The  availability  of  all  the  phosphate  fertilizers  varies  with  the 
fineness,  and  for  this  reason  all  the  samples  used  in  the  preceding 


30 

experiments  were  ground  to  as  nearly  the  same  degree  of  fineness 
as  possible.     This  applies  more  especially  to  the  insoluble  phosphates. 

(7)  Basic  slag  is  more  effective  as  a  source  of  phosphoric  acid  than 
phosphate  rock,  bone  meal,  or  reverted  phosphate. 

(8)  In  field  conditions  there  is  not  such  an  abundant  supply  of 
water  as  was  used  in  the  pots,  hence  the  soluble  phosphates  should  be 
relatively  more  effective  under  field  trials. 

(9)  Lime  applied  with  phosphates  temporarily  assists  the  plants  in 
assimilating  phosphoric  acid,  but  it  soon  loses  its  effectiveness  unless 
present  in  excessive  amounts. 

SOLUBILITY  OF   DIFFERENT  PHOSPHATES  IN  HAWAIIAN   SOILS. 

The  chemical  analyses  of  Hawaiian  soils  by  solution  in  hydro- 
chloric acid  of  specific  gravity  1.115  or  by  fusion  with  sodium  carbon- 
ate, with  scarcely  a  single  exception,  show  a  far  greater  amount  of 
phosphoric  acid  in  the  soil  than  would  be  required  for  plant  growth. 

Hawaiian  soils  are  largely  a  product  of  the  disintegration  of  basaltic 
lava  and  contain  abnormally  high  percentages  of  iron  and  aluminum. 
Likewise,  they  contain  high  percentages  of  phosphoric  acid,  but  in 
spite  of  this  fact,  plants  suffer  from  the  lack  of  phosphoric  acid, 
especially  in  the  red-clay  type  of  soil. 

In  the  work  herewith  presented,  fifth-normal  nitric  acid,  1  per  cent 
citric  acid,  1  per  cent  sodium  hydroxid,  hydrochloric  acid  (specific 
gravity  1.115),  water,  and  finally  fusion  with  sodium  carbonate  were 
used  in  determining  the  solubility  and  combinations  of  the  phosphates. 
While  the  voluminous  literature  regarding  the  action  of  these  reagents 
as  means  of  determining  the  availability  of  phosphoric  acid  is  very 
contradictory,  in  general  it  may  be  said  that  fifth-normal  nitric  acid 
acts  primarily  as  a  solvent  for  calcium  phosphate,  while  1  per  cent 
sodium  hydroxid  dissolves  the  iron  and  aluminum  phosphates. 

In  the  experiments  here  reported,  the  soils  were  treated  with  the 
solvents  as  follows :  Digested  with  hydrochloric  acid  (specific  gravity 
1.115)  according  to  the  official  method;  digested  with  1  per  cent  citric 
acid  for  3  days  with  occasional  shaking,  the  proportion  of  soil  to 
acid  being  1  to  10;  digested  with  fifth-normal  nitric  acid  for  5 
hours  at  40°  C.  in  the  same  proportion;  digested  with  1  per  cent 
sodium  hydroxid  for  5  hours  in  boiling  water  in  the  same  propor- 
tion, and  finally  digested  with  water  for  1  week  with  frequent 
shaking. 

SOLUBILITY   OF   PHOSPHORIC    ACID    IN    POTTED    SOIL. 

Following  the  removal  of  the  first  crop  of  millet  on  the  red-clay  soil 
(Experiment  I),  samples  of  soil  were  taken  from  all  of  the  pots  for 
analysis.     Distilled  water,  1  per  cent  citric  acid,  and  fifth-normal 


31 

nitric  acid  were  used  as  solvents.  One  hundred  grams  of  soil  was 
shaken  with  500  cubic  centimeters  of  water  for  1  week  and  250  cubic 
centimeters  used  for  analysis  by  the  colorimetric  method.  Fifty 
grams  of  soil  was  shaken  with  500  cubic  centimeters  of  1  per  cent 
citric  acid,  and  20  grams  of  soil  was  digested  and  shaken  with  200 
cubic  centimeters  of  fifth-normal  nitric  acid  at  40°  C.  for  5  hours. 
An  analysis  was  made  of  100  cubic  centimeters  of  the  nitric  acid 
extract,  from  which  no  precipitate  could  be  obtained  with  molybdate 
solution.  Only  a  trace  of  phosphate  could  be  detected  by  colorimetric 
determination  in  50  cubic  centimeters  of  this  solution.  The  results 
obtained  with  water  and  citric  acid  are  given  in  Table  VIII. 

Table  VIII. — Solubility  of  -phosphoric  acid  in  the  soils  in  pots  of  Experiment  I. 


Pot  Xos. 


Fertilizer  added. 


Soluble 
in  water. 

Soluble 

in  citric 

acid. 

Parts  per 

million. 

Per  cent. 

4.8 

0. 00280 

4.8 

.00310 

4.4 

.00335 

8.0 

.00350 

3.8 

.00444 

9.4 

.00508 

7.4 

.00584 

5.4 

.00537 

4.6 

. 00537 

3.0 

.00814 

3.0 

.00663 

6.4 

.00612 

7.6 

.00795 

4.8 

.00558 

6.8 

.00493 

4.8 

.00698 

5.2 

.00652 

6.0 

. 00730 

6.0 

.00809 

4.0 

.00665 

3.4 

.00557 

3.4 

.00649 

3.4 

.00493 

2.6 

.00728 

2.8 

.00255 

3.0 

.00270 

3.4 

.00255 

3.2 

.00303 

2.2 

.00222 

2.6 

.00255 

2.8 

.00715 

2.8 

.00525 

4.0 

.00780 

3.2 

.00413 

5.4 

.00255 

3.2 

.00431 

2.4 

.0127.-. 

3.2 

.00828 

2.4 

.00619 

4.8 

.00716 

2.6 

.00619 

2.6 

.00652 

6.8 

.00731 

5.8 

.00717 

1.2 

5,6 

9,10 

13.14 

17.1- 

21.22 

25.2>3 

29.30 

33,34 

41.42 

45.46 

49.50 

53.54 

57.58 

61.02 

65.1  i 

69.70 

73.71 

77.7s 

81.-2 

89,90 

93.94 

97,98 

99,102.... 
105,106... 
109.110... 
113,111... 
117.11-... 
121.122... 
125.126... 
129.130... 
133.134... 
137.13-... 
141.142... 
145.146... 
149,150... 
153.154... 
157.15-... 
161.162... 
166  166... 

i::;.i74... 


K-Ca-L-Pi/2  (slag  A) 

K-Ca-Pi/2  (slagA) 

K-Ca-L-P^  (slag  B) 

K-Ca-Pj/2  (slag  B) 

K-Ca-L-Pj/2  (slag  C) 

K-Ca-Pi/2  (slag  C) 

K-Ca-L-Pi/2  (slag  D) 

K-Ca-Pi/2  (slagD) 

K-Ca-L-Px/2  (acid  phosphate) . . . 

K-Ca-Pi/2  (acid  phosphate) 

K-Ca-L-P  i/j  (phosphate  rock) . . . 

K-Ca-Pi/2  (phosphate  rock) 

K-Ca-L-Pi/2  (sodium  phosphate) 
K-Ca-Pi/2  (sodium  phosphate) . . . 

K-Ca-L-P!  (slag  A) 

K-Ca-Pi  (slag  A) 

K-Ca-L-P!  (slag  B 

K-Ca-Pi  (slag  B) 

K-Ca-L-Px  (slag  C) 

K-Ca-Pi  (slag  C) 

K-Ca-L-P!  (slag  D) 

K-Ca-Pi  (slag  D) 

K-Ca-L-P!  (acid  phosphate) 

K-Ca-Pi  (acid  phosphate) 

K-Ca-L-Pi  (phosphate  rock) 


K-Ca-Pi  (phosphate  rock). 

Pi  (sodium  phosphate) . 


K-Ca-L-I 

K-Ca-Pi  (sodium  phosphate). 

K-Cax-L-Pi  (sodium  phosphate) 

K-Cax-Pi  (sodium  phosphate) 

1/2-K1 1/2-Ca-Iy-Pi  (sodium  phosphate). 
1/2-K1 1/2-Ca-Pi  (sodium  phosphate) . . . 
K-Ca-L-Pi  1/2  (sodium  phosphate) 


K-Ca-Pi  1/2  (sodium  phosphate) . 
P2  (phosphate  rock) 


K-Ca-L- 

K-Ca-P2  (phosphate  rock) 
K-Ca-L-Pi/2  (superphosphate) 
K-Ca-Pi/2  (superphosphate) . . 
K-Ca-L-Pi  (superphosphate).. 
K-Ca-Pi  (superphosphate) 


Owing  to  the  high  fixing  power  of  this  type  of  soil,  the  analysis  of 
the  water  extracts  would  not  be  expected  to  show  any  great  varia- 
tion in  solubility.     Such  proved  to  be  the  case,  and  it  is  probable 


32 

that  precipitation  or  fixation  from  the  extract  may  have  taken  place, 
since  the  process  of  extraction  was  extended  over  a  period  of  one 
week.  The  results  as  a  whole  indicate  the  impossibility  of  determin- 
ing any  increase  in  concentration  of  the  soil  solution  in  this  type  of 
soil  as  regards  phosphoric  acid  by  means  of  extraction  with  distilled 
water. 

The  results  from  treatment  with  citric  acid  were  somewhat  differ- 
ent. Practically  all  pots  to  which  phosphates  were  added  showed  a 
greater  solubility  in  this  solvent  than  the  check  pots.  Furthermore, 
the  solubility  was  more  or  less  dependent  upon  the  amount  of  phos- 
phate added  to  the  pots.  The  pots  showing  the  largest  amount  of 
phosphate  soluble  in  citric  acid  were  those  to  which  sodium  phos- 
phate had  been  added  at  the  rate  of  0.021  per  cent,  and  these  pots 
also  produced  the  best  growth. 

It  is  evident  from  the  results  obtained  by  extraction  with  fifth- 
normal  nitric  acid  that,  if  this  acid  is  a  solvent  for  calcium  phosphate, 
the  lime  added  had  little  influence  as  regards  combination  with  the 
phosphate,  since  the  acid  extracted  little  more  than  a  trace  of  phos- 
phate from  the  soil. 

SOLUBILITY  OF  PHOSPHATE  FERTILIZER  AFTER  ADDITION  TO  THE  SOIL. 

While  the  preceding  data  indicated  a  variation  in  the  solubility  of 
different  types  of  phosphate,  in  order  to  study  the  relation  more 
thoroughly  a  series  of  experiments  was  planned,  using  the  red-clay 
soil.  Nineteen  portions  of  soil  of  100  grams  each  were  weighed 
into  large  porcelain  dishes.  Duplicate  portions  of  this  soil  were 
treated  with  each  of  the  following  phosphates,  added  at  the  rate  of 
1  per  cent  P205:  Acid  phosphate,  superphosphate,  slag,  phosphate 
rock,  tripotassium  phosphate,  monosodium  phosphate,  disodium 
phosphate,  commercial  sodium  phosphate,  and  monocalcium  phos- 
phate. The  remaining  portion  was  used  as  a  check.  After  the 
addition  of  the  phosphate  the  soil  was  well  mixed  and  saturated  with 
water,  then  exposed  to  air  and  sunlight  to  dry  and  weather.  Satura- 
tion and  drying  was  repeated  twice.  Upon  the  third  drying,  after 
about  two  months'  time,  the  different  portions  of  soil  were  trans- 
ferred to  percolators,  and  700  cubic  centimeters  of  distilled  water  was 
allowed  to  percolate  through  each  sample.  A  separate  analysis  was 
made  of  each  100  cubic  centimeters  of  the  percolate.  The  results  are 
given  in  Table  IX. 


33 

Table  IX. — Phosphoric  acid  soluble  in  water. 
[Expressed  in  parts  per  million.] 


Percolates  of  100  cc.  each. 

Phosphate  added. 

First 
100. 

Second 
and 

third 
100. 

Fourth 
100. 

Fifth 
100. 

Sixth 
100. 

Seventh 
100. 

Total. 

26 
28 
36 
36 

Trace. 

Trace. 
4 
4 

Trace. 

Trace. 

Trace. 

Trace. 

Trace. 

Trace. 

Trace. 

Trace. 

62 

54 

3 

16 

20 

70 

66 

4 

4 

4 

4 

Trace. 

4 

4 

4 

38 

66 

4 

4 

26 
26 
60 
60 
6 
4 
4 
4 

38 
27 
58 
58 
6 
10 
4 
4 

11 

29 

46 

48 

6 

21 
16 
16 
21 
9 
6 
4 
4 

138 

Do 

146 

286 

Do 

289 

Slag 

31 

Do 

24 

Phosphate  rock 

4 
4 

24 

Do 

24 

Commercial  sodium  phosphate 

Do 

25 
25 
28 
•40 
60 
14 
16 
70 
95 
4 

14 
16 
10 

4 
48 
15 

8 
76 
70 

4 

22 
28 
22 
30 
29 
12 
16 
82 
76 

17 
7 
11 
18 
24 
9 
17 
21 
33 

82 

Disodium  phosphate 

80 

D  o . . .  t ! 

75 

Monosodium  phosphate 

130 

Do I " 

227 

Tripotassium  phosphate 

54 

'Do .1 ...' 

61 

Monn^alHum  phnsphato 

311 

Do ~....r 

100 
4 

428 

Check 

After  700  cubic  centimeters  had  passed  through,  the  soil  was 
removed  from  the  percolators  and  extracted  with  1  per  cent  citric 
acid  according  to  the  Dyer  method.  The  solubility  in  this  solvent 
is  given  in  Table  X. 

|Table  X. — Phosphoric  acid  soluble  in  1  per  cent  citric  acid. 
[Expressed  in  per  cent  of  air-dry  soil.] 


Phosphate  added. 

Per  cent. 

Phosphate  added. 

Per  cent. 

Acid  phosphate 

0.210 
.204 
.182 
.192 
.138 
.151 
.204 
.202 
.212 

Commercial  sodium  phosphate 

0.180 

Do 

Disodium  phosphate* r 

.154 

.242 

Do 

Do 

.198 

Slag 

Tripotassium  phosphate 

.124 

Do 

'Do ". * 

.122 

Phosphate  rock 

Monocalcium  phosphate 

.222 

Do  ...   . 

Do 

.222 

Check 

.019 

The  results  clearly  show  the  rapid  fixation  which  takes  place  when 
soluble  phosphates  are  added  to  the  soil  as  well  as  the  solubility  of 
the  phosphates  after  fixation. 

In  view  of  these  results,  a  further  set  of  experiments  was  planned 
in  order  to  study  the  comparative  action  of  iron  and  aluminum  phos- 
phates. If  soluble  phosphates  revert  to  iron  and  aluminum  phos- 
phates, it  is  desirable  to  study  the  solubility  of  the  latter.  This  series 
was  carried  out  in  the  same  manner  as  the  previous  one  except  that 
separate  samples  were  extracted  and  a  larger  quantity  of  soil — that 


34 

is,  25  grams  of  soil  to  250  cubic  centimeters  of  solvent — was  used. 
Additional  solvents  were  used  in  order  to  make  the  experiment  com- 
plete.    The  results  are  given  in  Table  XL 

Table  XI. — Solubility  of  phosphoric  acid,  added  at  the  rate  ofl  percent,  in  red-clay  soil. 


Fertilizer  added. 


Acid  phosphate 

Phosphate  rock 

Sodium  phosphate 

Tripotassium  phosphate 
Ferrous  phosphate 

Do 

Ferric  phosphate 

Do 

Check 


Soluble 
in  water. 


Per  cent. 

0.00C9 
.0027 
.0848 
.0249 

Trace. 

Trace. 

Trace. 

Trace. 

Trace. 


Soluble 
in  fifth- 
normal 
nitric 
acid. 


Per  cent. 
0.297 
.251 
.353 
.217 
.496 
.464 
.346 
.319 
Trace. 


Soluble 
in  l  per 
cent  cit- 
ric acid. 


Per  cent. 
0.237 
.181 
.304 
.152 
.385 
.380 
.235 
.208 
.0025 


Soluble 
in  1  per 
cent  so- 
dium hy- 
droxid. 


Per  cent. 
0.222 
.077 
.632 
.442 
.720 
.528 
.494 
.360 
.052 


These  results  indicate  that  a  large  part  of  the  soluble  phosphates 
does  revert  to  the  iron  and  aluminum  phosphates. 


SOLUBILITY     OF     PHOSPHATE     NATURALLY      OCCURRING     IN     HAWAIIAN 

SOILS. 

Table  XII,  showing  the  relatively  small  quantities  of  phosphoric  acid 
dissolved  out  of  Hawaiian  soils  by  fifth-normal  nitric  acid,  which  is  a 
solvent  of  calcium  phosphate,  and  the  larger  quantities  soluble  in  1 
per  cent  sodium  hydroxid,  which  is  a  solvent  of  iron  and  aluminum 
phosphates,  indicates  that  the  major  part  of  the  phosphates  in  these 
soils  exists  normally  in  combination  with  iron  and  aluminum. 

Table  XII. — Solubility  of  phosphoric  acid  in  various  types  of  Hawaiian  soils. 
[Expressed  as  per  cent  r2Of,.] 


Soil     Soil 
No.  1.  No.  2. 

Soil 
No.  3. 

Soil 
No.  4. 

Soil 
No.  5. 

Soil 
No.  6. 

Soil 
No.  7. 

Soil 
No.  8. 

Soil 
No.  9. 

Soil 
No. 
10. 

Soil 
No. 
11. 

Hydrochloric  acid  (specific  gravity 
1.115) 

0.677,0.28 
.152    -0036 

0.289 
.0033 
.0018 
.0615 

.710 

2.171 

0.427 

0.286 
.005 
.0003 
.279 

.460 

0.116 
.004 
.0004 
.052 

.240 

0.104 
.003 
.0003 
.055 

.450 

0.024 

0.234 

0.225 

.324    -008 

.004 

Fifth-normal  nitric  acid 

One  per  cent  sodium  hydroxid 

Total  phosphoric  acid  as  deter- 
mined by  fusion  with  sodium 

.007 
.153 

1.060 

.0024 
.0363 

.670 

.015 
.222 

3.300 

.0005 
.298 

.440 

.0001 
.008 

.030 

.0013 
.008 

.400 

.0005 
.043 

.340 

Phosphoric  acid  in  humus,  per 

22.98 
1.49 

1.09 
3.48 

1.02 
4.93 

4.96 
7.96 

2.98 
3.83 

2.38 
3.93 

1.23 
3.50 

1.36 

3.78 

0.530 
1,690 

1.110 

0.930 

Humus  in  soil per  cent. . 

1.630 

6.960 

In  selecting  the  soils  to  be  tested,  several  widely  varying  types  were 
chosen.  No.  1  is  a  sandy  soil  high  in  magnesia  (8.74  per  cent)  and 
lime  (1.84  per  cent)  and  is  the  same  as  No.  1  in  the  pot  experiments, 
which  did  not  respond  to  phosphate  fertilizers ;  Nos.  2  and  3  are  the 


35 

same  as  in  the  pot  experiments;  No.  4  is  a  very  productive  silt,  hi^h 
in  lime  (3.80  per  cent)  and  humus;  Nos.  5  and  6  are  brown-clay  soils 
belonging  to  the  class  of  clays  which  contain  a  larger  amount  of  iron 
than  aluminum;  Nos.  7  and  8  are  clay  soils  belonging  to  the  class  of 
clays  which  show  a  higher  content  of  aluminum  than  iron;  No.  9  is 
a  soil  containing  20  per  cent  titanium,  about  40  per  cent  iron  oxid, 
and  8  per  cent  aluminum;  No.  10  is  a  soil  which  is  principally  coral 
sand  (about  90  per  cent  calcium  carbonate);  No.  11  is  a  sandy  soil 
from  a  humid  district  and  is  high  in  both  lime  (6.3  per  cent)  and  mag- 
nesia (5.8  per  cent).  These  9  types  of  soils  include  all  the  important 
ones  of  the  islands,  for  which  reason  the  data  should  be  of  wide 
application  in  drawing  conclusions  regarding  the  locking  up  of  the 
phosphate. 

The  data  given  in  the  table  show  that  there  are  included  soils 
which  possess  all  the  conditions  generally  considered  essential  for  the 
fixation  of  phosphates.  There  are  the  normal  conditions,  such  as 
high  clay  content,  colloidal  clay,  and  organic  compounds,  which  pro- 
mote physico-chemical  absorption;  the  humic  conditions  which  pro- 
mote biological  absorption;  and,  finally,  the  chemical  conditions, 
such  as  high  content  of  lime,  magnesium,  iron,  aluminum,  and  tita- 
nium, which,  either  through  an  actual  combination  or  a  reversion  to 
a  less  soluble  form,  influence  chemical  fixation.  At  least  one  or 
possibly  all  three  of  the  above  factors  may  influence  the  maintenance 
of  a  favorable  medium  in  the  soil  for  plant  growth,  in  so  far  as  plant 
growth  is  affected  by  the  presence  of  a  readily  available  source  of 
phosphoric  acid.  A  relation  may  be  established  from  data  given  in 
Table  XII  between  the  chemical  and  physical  composition  of  the  soil, 
the  solubility  of  phosphoric  acid  in  various  solvents,  and  its  availa- 
bility as  measured  by  plant  growth. 

Table  XII  shows  the  relative  solubility  of  phosphoric  acid  in  all  the 
important  soil  types  of  the  Hawaiian  Islands.  Since  three  of  these 
types,  Nos.  1,  2,  and  3,  were  used  in  the  pot  experiments,  a  com- 
parison of  the  data  will  indicate  the  relation  between  the  solubility 
and  the  availability  as  measured  by  the  growth  of  millet.  Soil  No.  1 
did  not  respond  to  phosphate  fertilizer,  thus  showing  the  high  avail- 
ability of  its  phosphoric  acid;  soil  No.  2  was  greatly  in  need  of  avail- 
able phosphates,  as  indicated  by  the  marked  increase  in  plant  growth 
following  the  application  of  phosphate  fertilizers  in  all  forms;  soil 
No.  3  was  loss  in  need  of  phosphate  than  No.  2,  as  indicated  by  a 
smaller  increase  resulting  from  the  addition  of  phosphate.  The 
other  soils  have  not  been  used  in  pot  experiments.  The  results  of 
previous  experiments  '  with  soils  of  the  same  type  as  certain  of  those 
included  in  Table  XII  are  summarized  in  Table  XIII. 

I  W.  P.  Keller,  Jour.  Indus,  and  Kn-in.  (hem.,  2  (1910),  p.  277. 


36 


Table  XIII. — Solubility  and  availability  of  phosphoric  acid  in.  ferruginous  soils. 


Soil  No. 

Soluble  in 

fifth- 
normal  hy- 
drochloric 

acid. 

Soluble  in 

1  per  cent 

sodium 

hydroxid. 

Crop  grown. 

Crop 

increase 

from 

phosphate. 

10 

Per  cent. 
Trace. 
Trace. 
Trace. 
0. 1128 

Per  cent. 

0.0060 

.0219 

.0089 

.1858 

Pineapples  — 

.do 

Cotton 

Per  cent. 
50 

11 

100 

12 

200 

13 

Soils  Nos.  10,  11,  and  12  are  samples  of  red  soil  of  the  type  used  in 
the  pot  experiments,  responding  to  phosphate  fertilization,  while  No. 
13  is  a  sample  of  the  same  soil  used  as  No.  1  in  the  pot  experiments. 
Hence  it  may  be  seen  that  the  results  are  directly  in  accord  with  those 
presented  in  this  bulletin. 

Table  XII  shows  clearly  the  insoluble  nature  of  the  phosphoric  acid 
in  the  different  types  of  soil.  Fusion  with  sodium  carbonate  shows 
the  total  phosphate  content  to  be  very  high  in  practically  all  types. 
The  results  of  the  official  method  of  extraction  with  hydrochloric  acid 
(specific  gravity  1.115)  throw  considerable  discredit  upon  its  useful- 
ness as  a  means  of  determining  the  phosphate  content  of  Hawaiian 
soils.  This  may  be  attributed  to  several  causes,  chief  among  which 
is  the  inability  of  the  acid  to  penetrate  to  such  an  extent  during  the 
period  of  extraction  as  to  come  in  contact  with  any  occluded  crystals 
or  other  protected  particles  of  phosphate,  and,  furthermore,  its  lack 
of  ability  thoroughly  to  decompose  the  basic  phosphates  of  iron, 
aluminum,  and  titanium,  especially  the  last.  The  obvious  conclusion 
to  be  drawn  from  these  results  is  the  uselessness  of  determining  the 
phosphoric  acid  in  the  hydrochloric  acid  extract  and  the  need  of 
determining  the  absolute  phosphate  content. 

The  action  of  weak  solvents  upon  the  various  types  of  Hawaiian 
soils  is  a  means  of  obtaining  data  of  value  regarding  the  solubility  of 
phosphoric  acid,  but  not  regarding  its  availability.  Neither  is  it 
possible  to  determine  in  this  way  whether  the  soil  will  respond  to 
phosphate  fertilization.  Stoddard1  says  that,  for  Wisconsin  at  least, 
if  a  soil  contains  less  than  0.015  per  cent  of  phosphoric  acid  soluble  in 
fifth-normal  nitric  acid,  it  will  respond  to  phosphate  fertilization. 
Snyder,2  working  independently,  reached  the  same  conclusion  in 
regard  to  Minnesota  soils.  That  such  a  relation  holds  good  for 
Hawaiian  soils  does  not  at  this  time  appear  probable,  primarily  be- 
cause there  are  too  many  other  abnormal  factors  to  be  considered. 
Soil  No.  1  has  been  shown  not  to  respond  measurably  to  phosphate 
applications,  yet  it  contains  only  0.007  per  cent  phosphoric  acid 
soluble  in  fifth-normal  nitric  acid,  but,  on  the  other  hand,  contains 


Wisconsin  Sta.  Research  Bui.  2  (1909). 


2  Minnesota  Sta.  Bui.  102  (1907),  p. 


37 

0.153  per  cent  soluble  iron  and  aluminum  phosphates.  The  weak 
solvents  apparently  exert  a  very  limited  solvent  action,  as  is  shown 
by  the  small  percentage  of  the  total  phosphoric  acid  dissolved.  All 
the  soils,  irrespective  of  type,  contain  iron  and  aluminum  phosphates 
far  in  excess  of  calcium  phosphates.  Citric  acid  is  a  much  more 
effective  solvent  than  nitric  acid. 

The  humus  content  of  Hawaiian  soils  varies  between  wide  limits 
according  to  locality  and  climatic  conditions.  Since  humus  is  a 
factor  in  the  availability  of  phosphoric  acid,  some  determinations 
were  made  of  the  humus  content  of  various  types  and  also  of  the 
phosphoric  acid  combined  with  the  humus.  There  is  apparently  no 
relation  between  the  amount  of  humus  in  the  soil  and  the  phosphate 
content  of  the  humus.  Stoddard  1  found  that  as  the  amount  of  humus 
decreases  the  percentage  of  phosphoric  acid  in  the  humus  increases. 
This  is  not  true  of  Hawaiian  soils,  but,  as  a  general  rule,  it  may  be 
said  that  those  high  in  lime  or  magnesia  and  humus  contain  a  large 
percentage  of  phosphoric  acid  organically  combined.  Attention  is 
called  to  the  high  phosphate  content  of  soil  No.  1,  in  which  the  humus 
contains  22.98  per  cent  phosphoric  acid.  In  view  of  the  fact  that 
this  soil  has  been  found  not  to  respond  to  phosphate  fertilization  and 
yet  to  have  a  very  low  content  of  calcium  phosphate,  as  measured  by 
fifth-normal  nitric  acid,  the  conclusion  is  evident  that  the  organic 
phosphate  in  this  soil  is  present  in  a  readily  available  form. 

The  soils  having  the  strongest  fixing  power  are  likewise  those  con- 
taining phosphoric  acid  in  least  available  form  as  measured  by  plant 
growth  and  also  those  containing  the  least  phosphoric  acid  soluble  in 
weak  solvents.  These  phosphates  are  only  slightly  soluble  in  citric 
and  nitric  acids,  but  are  more  soluble  in  weak  alkali.  The  soils  of 
lowest  fixing  power,  due  primarily  to  lower  clay  content,  are  high  in 
lime  and  magnesia.  These  soils  are  also  high  in  phosphate,  and  while 
a  large  percentage  is  in  the  form  of  iron  and  aluminum  phosphate,  the 
proportion  soluble  in  citric  acid  is  equal  to  or  more  than  that  soluble 
in  alkali.  An  exception  to  this,  and  one  to  which  attention  should  be 
called,  is  soil  No.  11.  This  soil  is  high  in  lime,  magnesia,  and  humus, 
but  was  taken  from  a  very  humid  district.  The  analysis  shows  the 
major  part  of  the  calcium  phosphate  to  have  been  washed  out,  leaving 
the  iron  and  aluminum  phosphates.  This  is  probably  similar  to  the 
action  of  weathering  agents  upon  lava  in  the  original  Hawaiian  soil 
formation,  as  a  result  of  which  lime  has  decreased  from  about  10  per 
cent  to  less  than  1  per  cent  in  the  average  soils.  The  phosphoric  acid 
has  decreased  quite  often  also  during  this  process  of  disintegration. 

Soils  Nos.  2,  3,  5,  6,  7,  and  8  are  all  clay  soils.  Nos.  5,  6,  7,  and  8 
were  chosen  for  the  fact  that  in  the  two  former  the  iron  content  is  in 

i  Wisconsin  Sta.  Research  Bui.  2  (1909). 


38 

excess  of  the  aluminum,  while  in  the  two  latter  the  opposite  relation 
exists.  If  it  is  permissible  to  assume  that  the  phosphate  is  largely  in 
combination  with  iron  in  soils  Nos.  5  and  6,  while  that  in  Nos.  7  and 
8  is  largely  in  combination  with  aluminum,  then  the  former  is  more 
soluble  and,  as  results  have  shown,  slightly  more  available.  On  the 
other  hand,  Hawaiian  clay  soils,  in  which  the  aluminum  content  is 
higher  than  the  iron,  possess  more  marked  colloidal  properties,  and 
hence,  due  to  their  physical  influences,  should  contain  phosphoric 
acid  in  a  less  soluble  and  less  available  form. 

It  appears  that  the  solubility  of  the  phosphates  already  present  in 
the  soil  is  dependent  partly  upon  conditions  of  equilibrium  which 
influence  the  concentration  of  the  soil  solution.  One  effect  of  an 
added  fertilizer  is  to  disturb  this  state  of  equilibrium.  For  this  reason 
the  solubility  or  availability  of  an  added  phosphate  will  depend  upon 
its  action  following  its  addition  to  the  soil,  and  hence  is  influenced  by 
several  factors. 

Reference  to  Tables  IX,  X,  and  XI  will  clearly  show  that  the  solu- 
bility of  the  phosphates  before  adding  to  the  soil  can  not  be  used  as  a 
criterion  of  their  solubility  after  addition.  According  to  the  results 
tabulated  in  Table  IX,  fixation  is  apparently  influenced  by  the  valency 
of  the  salt.  Trivalent  potassium  phosphate  is  fixed  most  strongly, 
divalent  sodium  phosphate  next  strongest,  while  monovalent  sodium 
phosphate  is  most  soluble.  Superphosphate  is  much  more  soluble 
than  the  sodium  and  potassium  phosphates,  as  is  also  acid  phos- 
phate, with  the  exception  of  monosodium  phosphate.  These  facts 
indicate  that  the  fixing  or  reversion  of  the  calcium  phosphate  is  much 
less  rapid  than  that  of  the  sodium  and  potassium  salts,  or  possibly 
that  the  calcium  salt  is  not  so  strongly  fixed.  But  this  does  not  cor- 
relate with  the  availability  as  measured  by  the  plant  growth  in  this 
soil,  where  sodium  phosphate  was  the  most  effective  phosphate 
fertilizer.  The  sodium  and  potassium  salts  were  least  soluble  in  the 
first  portion  of  water  passing  through  the  soil,  and  in  this  they  dif- 
fered radically  from  the  calcium  salts.  This  indicates  the  possible 
influence  of  physical  factors  upon  their  solubility.  Phosphate  rock 
and  slag  proved  to  be  the  least  soluble  in  water. 

The  solubility,  as  measured  by  citric  acid  (Table  X),  indicates  that 
citric  acid  only  magnifies  the  action  of  water.  All  forms  of  calcium 
phosphate  were  among  the  most  soluble,  except  Thomas  slag  (tetra- 
calcium  phosphate).  The  solubility  in  citric  acid  is  also  influenced 
by  the  valency  of  the  sodium  and  potassium  phosphate,  namely, 
the  monobasic  phosphate  is  most  soluble  while  the  tribasic  is  least 
soluble. 

Somewhat  more  complete  data  are  given  in  Table  XI.  The  solu- 
bility in  water  is  slightly  different  from  that  given  in  the  previous 


39 

table,  due  to  a  difference  in  method  of  extraction.  It  may  bo  safely 
said  that  the'  method  of  percolation  more  nearly  represents  soil 
conditions.  The  motive  in  using  several  solvents  was  to  determine 
the  combinations  formed  in  the  soil  by  the  phosphates.  The  data 
show  sodium  and  potassium  phosphates  to  be  readily  converted  into 
iron  and  aluminum  phosphates.  These  salts  also  show  a  high  solubility 
in  fifth-normal  nitric  acid,  indicating  either  a  residue  unconverted 
as  shown  in  column  3,  or  a  partial  conversion  to  the  soluble  calcium 
salt,  which  would  be  the  only  calcium  salt  possible  of  formation  in 
the  absence  of  an  excess  of  lime.  The  iron  phosphates  proved  to  be 
quite  soluble  in  fifth-normal  nitric  acid,  in  fact,  to  a  greater  degree 
than  the  calcium  salts,  acid  phosphate,  and  phosphate  rock.  Tri- 
potassium  phosphate  is,  in  all  solvents,  less  soluble  than  disodium 
phosphate. 

It  may  be  said,  then,  that  soluble  phosphates,  when  added  to 
Hawaiian  soils,  combine  with  iron  and  aluminum  to  a  greater  degree 
than  with  calcium,  even  in  soils  containing  a  high  percentage  of  the 
last.  But  this  chemical  combination  in  itself  does  not  explain  the 
unavailable  nature  of  phosphates  in  the  soils,  as  is  shown  in  the 
results  obtained  from  both  the  pot  experiments  and  the  solubility 
experiments  noted  in  the  latter  half  of  this  bulletin.  The  unavail- 
able condition  is  brought  about  through  physico-chemical  activities, 
and  is  more  rapid  when  a  sodium  or  potassium  phosphate  is  added, 
because  of  the  rapid  deflocculation  of  the  clay,  which  causes  more 
complete  dissemination  of  the  salts. 

As  a  means  of  measuring  solubility,  all  solvents  used  are  of  more  or 
less  value.  Solubility  can  hardly  be  considered  a  measure  of  availa- 
bility except  in  so  far  as  a  comparison  of  the  solubility  in  several  of  the 
solvents  will  indicate  the  form  in  which  the  phosphoric  acid  is  com- 
bined in  the  soil.  As  the  above  data  show,  the  results  obtained  by 
the  use  of  the  different  solvents  are  quite  conflicting  On  certain  soils. 

At  the  beginning  of  this  work  the  most  plausible  theory  suggested 
to  explain  the  unavailability  of  the  phosphoric  acid  in  Hawaiian  soils 
seemed  to  be  its  possible  combination  with  titanium.  While  it  is 
undoubtedly  true  that  the  phosphoric  acid  may  be  present  to  a 
certain  extent  in  this  form,  which  is  highly  insoluble,  this  fact  is  of 
minor  importance  in  explaining  the  low  availability,  although  it  is 
perhaps  the  most  serious  chemical  factor.  Titanium  is  widely  dis- 
tributed in  Hawaiian  soils,  which  contain,  on  the  average,  from 
5  to  10  per  cent  of  titanium  oxid,  and  as  much  as  34  per  cent  has  been 
found.  If  this  constituent  were  to  bo  considered  a  prime  factor  in 
the  availability  of  the  phosphoric  acid,  it  would  bo  expected  that 
the  titanium  soils  would  have  a  high  phosphate  content,  due  to  reten- 
tion of  the  phosphoric  acid  as  titanium  phosphate.     The  analysis  of 


40 

this  type  of  soil,  as  shown  in  Table  XIV,  can  hardly  be  said  to  sup- 
port this  theory. 

Table  XIV. — Partial  analysis  of  titanium  soils. 


SoU  No. 

Titanium 

oxid 

(Ti02). 

Phos- 
phoric 
acid 
soluble 
in  hydro- 
chloric 
acid. 

Total 
phos- 
phoric 
acid. 

Soil  No. 

Titanium 

oxid 

(TiO»). 

Phos- 
phoric 
acid 
soluble 
in  hydro- 
chloric 
acid. 

Total 
phos- 
phoric 
acid. 

33 

Per  cent. 
21.90 
28.  G4 
27.49 
18.90 

Per  cent. 

0.04 

.05 

.09 

.07 

Per  cent. 

0.22 

.56 

.34 

.08 

165 

Per  cent. 
20.62 
18.84 
20.02 
34.16 

Per  cent. 

0.04 

.11 

.11 

.04 

Per  cent. 
0.17 

72 

174 

.50 

113 

175 

.27 

164 

199 

.35 

Titanium  and  total  phosphoric  acid  were  determined  by  fusion 
with  sodium  carbonate. 

The  above  figures  indicate  the  insolubility  of  titanium  phosphate, 
but  also  show  the  low  phosphate  content  of  this  type  of  soil.  The 
average  phosphate  content  of  Hawaiian  soils  is  about  0.5  per  cent 
P205,  while  the  maximum  is  2.32  per  cent  (determined  with  hydro- 
chloric acid  of  specific  gravity  1.115),  and  the  maximum  absolute 
phosphate  content  as  determined  by  fusion  with  sodium  carbonate  is 
3.50  per  cent.  It  appears,  therefore,  that  the  major  part  of  the 
phosphoric  acid  combined  with  the  titanium  in  the  lava  rock  remains 
as  such  in  the  soil  and  represents  the  chemically  combined  insoluble 
phosphate,  but  owing  to  the  inert  properties  of  the  titanium  as  com- 
pared with  the  other  elements  present  in  the  soil,  it  may  be  considered 
to  be  inactive  toward  other  phosphates  already  present  or  any  which 
may  be  added  as  fertilizers. 

SUMMARY. 

(1)  Hydrochloric  acid  of  official  strength  does  not  dissolve  all  of 
the  phosphoric  acid  of  Hawaiian  soils.  To  determine  the  total 
phosphate  content,  it  is  necessary  to  fuse  the  soil  with  sodium  car- 
bonate. 

(2)  Fifth-normal  nitric  acid  has  very  little  solvent  action  upon  the 
phosphate  in  the  soils,  indicating  the  absence  of  appreciable  quantities 
of  calcium  phosphate. 

(3)  One  per  cent  citric  acid  has  a  much  stronger  solvent  action  than 
fifth-normal  nitric  acid. 

(4)  Of  the  weaker  solvents,  1  per  cent  sodium  hydroxid  is  the 
strongest,  due  to  its  action  on  the  iron  and  aluminum  phosphates. 

(5)  The  fertilizer  (phosphate)  requirement  of  the  soil  is  not  meas- 
ured by  solubility  in  water  or  fifth-normal  nitric  acid,  but  it  may  be 
indicated  by  the  solubility  in  citric  acid. 


41 

(6)  The  solubility  of  a  phosphate  before  it  is  added  to  a  soil  can  not 
be  used  as  a  criterion  of  its  solubility  after  addition,  but  it  may  indi- 
cate its  availability. 

(7)  The  fixation  of  a  soluble  phosphate  by  the  soil  may  be  influenced 
by  the  basicity  of  the  soil. 

(8)  Availability  as  determined  with  solvents  does  not  agree  in  full 
with  that  indicated  by  plant  growth. 

ACKNOWLEDGMENT. 

Acknowledgments  are  due  and  are  hereby  extended  to  Dr.  W.  P. 
Kelley,  formerly  chemist  at  this  station,  under  whose  supervision 
this  work  was  inaugurated. 


APPENDIX. 


THE  DETERMINATION  OF  PHOSPHORIC  ACID  IN  HAWAIIAN  SOILS. 

During  the  course  of  a  great  many  soil  analyses  in  this  laboratory, 
it  has  been  found  that  influences  of  an  inhibitory  nature  seriously 
affect  the  determination  of  phosphoric  acid  in  the  hydrochloric  acid 
extract  of  Hawaiian  soils.  The  error  resulting  has  always  been 
attributed  to  the  presence  of  titanium,  although  no  work  has  been 
done  to  establish  this  theory  definitely. 

The  nature  of  the  error  is  indicated  by  the  appearance  of  a  white 
precipitate  upon  dissolving  the  yellow  phosphomolybdate  precipitate 
in  ammonia.  A  part  of  this  white  precipitate  may  pass  through  the 
filter  paper  and  appear  in  the  filtrate  in  a  flocculent  form  which  settles 
out  on  standing  overnight,  or  it  may  be  precipitated  before  passing 
through  the  filter  paper  and  hence  mask  the  appearance  of  the  error. 
In  many  soil  extracts  it  is  present  in  appreciable  amounts,  produc- 
ing seriously  misleading  results.  On  dissolving  the  yellow  precipi- 
tate on  the  paper  by  treating  first  with  hot  water  and  then  making 
alkaline  on  the  filter  with  ammonia,  a  larger  precipitate  is  obtained 
than  if  it  were  dissolved  by  a  hot  solution  of  ammonia. 

The  size  of  the  error  due  to  this  white  precipitate  is  indicated  by 
the  data  in  Table  XV  showing  the  difference  in  phosphate  content 
of  f  12  Hawaiian  soils  as  determined  by  the  official  method.  In  one 
series  the  precipitate  was  removed  before  adding  the  magnesia 
mixture,  in  another  the  magnesia  mixture  was  added  directly. 

Table  XV. —  Variation  in  phosphoric  acid  content  as  determined  by  different  methods. 


Method. 

Soil 
No.  1. 

Soil 
No.  2. 

Soil 
No.  3. 

Soil 
No.  4. 

Soil 
No.  5. 

Soil 
No.  6. 

Soil 
No.  7. 

Soil 
No.  8. 

Soil 
No.  9. 

Soil 
No. 
10. 

Soil 
No. 
11. 

Soil 
No. 
12. 

Filtered 

P.c. 

0.299 
.395 
.175 

P.c. 

0.311 

.399 

.188 

P.c. 

0.346 
.419 
.224 

P.c. 

0.304 
.349 
.227 

P.c. 

0.412 

.596 

.204 

P.c. 

0.330 

.495 

P.c. 

0.280 
.446 

P.c. 

0.526 

.733 

.569 

P.c. 

0.515 
.824 
.514 

P.c. 

0.277 
.445 
.219 

P.c. 

0.302 
.402 
.240 

0.44^ 

Not  filtered 

.607 

.382 

As  indicated  in  the  table,  the  error  varies  from  0.073  per  cent  to 
0.309  per  cent,  depending  on  whether  the  white  precipitate  is  removed 
by  filtration  or  weighed  as  magnesium  pyrophosphate.  The  volumet- 
ric method  of  titrating  the  yellow  phosphomolybdate  with  standard 
alkali  does  not  eliminate  the  error,  but  for  comparison,  the  deter- 
mination by  this  method  is  given  in  the  table. 

(42). 


43 


Among  the  elements  which  must  be  considered  as  having  the  great- 
est influence  upon  the  determination  of  phosphoric  acid  in  soils  are 
iron,  aluminum,  titanium,  and  silicon.  Hence,  in  choosing  soils 
for  use  in  the  present  investigation,  only  those  showing  certain 
peculiarities  were  selected.  Partial  analyses  of  the  soils  are  given  in 
Table  XVI. 

Table  XVI . — Partial  analysis  of  soils  by  hydrochloric  acid  (specific  gravity  extraction 

1.115). 


Soil 

No. 

1. 

Soil 

No. 

2. 

Soil 

No. 

3. 

Soil 
No. 

4. 

Soil 

No. 

5. 

Soil 

No. 

6. 

Soil 
No. 

7. 

Soil 
No. 

8. 

Soil 

No. 

9. 

Soil 
No. 
10. 

Soil 
No. 
11. 

Soil 
No. 
12. 

P.c. 
14.84 
18.20 

1.40 
35.21 

P.c. 

13.28 

26.25 

1.40 

30.52 

P.c. 

15.72 

24.78 

1.80 

29.64 

P.c. 
11.60 
26.01 

1.20 

28.57 

P.c. 
15.16 
19.55 

2.20 
29.22 

P.c. 

19.32 

21.76 

3.00 

30.57 

P.c. 

25.26 
7.03 
5.40 

33.54 

P.c. 

25.28 
17.43 
5.20 
29.20 

P.c. 
31.00 

P.c. 

25.  20 

P.c. 

30.56 

11.94 

5.20 

33.35 

P.c. 
30.84 

Aluminum  oxid 

Titanium  oxid 

15.81  !l9.C3 

5.00     4. 20 
28. 65   27. 03 

10.68 
6.20 

Insoluble  residue 

Since  the  insoluble  condition  of  phosphates  in  Hawaiian  soils  is 
partly  due  to  physical  influences,  only  soils  of  the  red-clay  types  were 
used  in  this  work.  Soils  Nos.  1  to  6,  inclusive,  represent  the  type  of 
highly  colloidal  soil  in  which  the  aluminum  content  is  in  excess  of  the 
iron.  In  Nos.  7  to  12,  inclusive,  the  opposite  relation  exists  between 
the  iron  and  aluminum.  These  relationships  influence  to  a  consider- 
able extent  both  the  physical  and  chemical  properties  of  the  soil,  and 
in  the  latter  type,  that  is,  those  in  which  the  iron  is  in  excess,  the 
formation  of  the  white  precipitate  is  more  prevalent.  It  should  also 
be  mentioned  that  the  latter  are  also  higher  in  titanium  than  the 
former.  An  analysis  of  this  white  precipitate  shows  it  to  contain 
about  25  per  cent  of  titanium  oxid,  70  per  cent  of  iron  and  aluminum 
oxids,  small  amounts  of  phosphoric  acid,  and  no  silica.  Hence  an 
error  will  result  in  the  determination  whether  the  precipitate  is  filtered 
off  or  weighed  as  magnesium  pyrophosphate.  The  only  solution  of 
the  problem  is  a  prevention  of  the  deposition  of  the  white  precipitate 
or  a  removal  of  the  inhibiting  factors.  That  titanium  and  iron, 
primarily  the  former,  are  the  elements  most  active  toward  its  forma- 
tion may  be  seen  by  reference  to  the  table  of  analyses  and  to  the 
table  showing  the  weight  oi  white  precipitate  deposited.  The  greatest 
error  occurs  in  those  soils  containing  the  highest  percentage  of  iron 
and  titanium,  but  the  total  elimination  of  these  elements  in  the  soil 
extract  without  removing  traces  of  phosphate  is  a  practical  impos- 
sibility. 

A  method  for  removing  silica  and  titanium  from  the  soil  extract 
recommended  by  the  Association  of  Official  Agricultural  Chemists  is 
to  evaporate  the  extract  to  dryness  and  take  up  in  hydrochloric  acid. 
This  method  fails  utterly  when  applied  to  local  soils  and  is  never  used 
in  this  laboratory.     Instead,  all  soil  analyses  are  made  directly  upon 


44 

the  acid  extract  after  oxidizing  the  iron  and  organic   matter  with 
nitric  acid. 

In  order  to  illustrate  the  loss  in  phosphoric  acid,  as  well  as  other 
elements,  resulting  through  an  evaporation  of  the  soil  extract  to 
dryness,  the  following  table  is  inserted.  Fifty  cubic  centimeters  of 
each  extract,  representing  1  gram  of  soil,  was  evaporated  to  dryness, 
taken  up  in  hydrochloric  acid,  filtered,  phosphoric  acid  determined 
in  the  filtrate,  and  the  insoluble  residue  analyzed. 

Table  XVII. — Composition  of  insoluble  residue  formed  on  evaporating  soil  extract  to 

dryness. 


Soil 

No. 

1. 

Soil 

No. 

2. 

Soil 

No. 

3. 

SoU 

No. 
4. 

Soil 

No. 

5. 

Soil 
No. 

6. 

Soil 

No. 

7. 

Soil 
No. 

8. 

Soil 

No. 

9. 

Soil 
No. 
10. 

Soil 

No. 
11. 

Soil 
No. 
12. 

Total  residue 

P.c. 

0.27 
.23 
C1) 

Tr. 
Tr. 
(l) 

P.c. 

0.24 
.22 

0) 
Tr. 
Tr. 
C1) 

P.c. 

0.14 

.14 

C1) 

Tr. 
Tr. 
C1) 

P.c. 

0.12 

.12 

Tr. 
Tr. 
0) 

P.c. 

0.12 

.12 

C1) 
Tr. 
Tr. 
C1) 

P.c. 

0.01 

.01 

0) 
Tr. 
Tr. 

C1) 

P.c. 

1.55 
.00 

1.05 
.18 
.10 
.32 

P.c. 

2.66 
.00 

1.85 
.22 
.67 
.59 

P.c. 

2.32 

.00 

1.50 

■  .18 
.20 
.64 

P.c. 

5.50 
.08 

4.30 
.33 
.15 
.71 

P.c. 

3.58 
.00 

2.35 
.18 
.12 

1.05 

P.c. 
3  06 

Silica 

22 

Titanium  oxid 

2  00 

Iron  oxid 

.18 

Phosphoric  acid 

Aluminum  oxW.. 

.08 
.44 

1  Not  determined. 

While  this  method  does  eliminate  the  formation  of  the  white  pre- 
cipitate and  indicates  the  primary  source  of  error  to  be  titanium, 
all  possibility  of  its  use  as  a  method  for  determining  phosphoric 
acid  in  the  soil  extract  is  precluded  by  the  results  in  the  above  table. 
Attention  is  called  to  the  remarkable  difference  in  the  properties  of 
the  two  chemical  types. 

After  a  thorough  trial  of  several  methods  and  modifications  the 
elimination  of  the  white  precipitate  was  successfully  effected  by  a 
precipitation  in  a  nitric-acid  solution  free  from  chlorids.  The  fol- 
lowing method  has  been  adopted : 

To  50  cubic  centimeters  of  the  hydrochloric-acid  extract,  repre- 
senting 1  gram  of  soil,  add  1  cubic  centimeter  of  nitric  acid  and  boil 
to  oxidize  the  organic  matter  and  ferrous  iron.  Add  ammonium  hy- 
droxid  until  faintly  alkaline,  boil  to  remove  excess  of  ammonia,  filter, 
and  wash  free  of  chlorids.  Transfer  filter  and  contents  to  the  original 
beaker,  add  an  excess  of  dilute  nitric  acid,  and  heat  to  boiling  on  the 
hot  plate  in  a  covered  beaker.  This  procedure  is  necessary  to  dissolve 
portions  of  the  ammonia  precipitate  which  assumes  a  colloidal  form  on 
boiling  in  an  ammoniacal  solution  insoluble  in  cold  nitric  acid.  In 
case  too  large  an  excess  of  nitric  acid  has  been  added,  it  should  be 
nearly  neutralized  with  ammonia,  several  grams  of  ammonium  nitrate 
added,  the  solution  diluted  to  100  to  150  cubic  centimeters,  and  50 
cubic  centimeters  of  molybdate  solution  added  slowly  while  stirring. 
The  beaker  is  then  placed  in  a  water  bath  at  55°  C.  for  four  hours. 
Further  procedure  is  the  same  as  that  of  the  official  method.1 

i  U.  S.  Dept.  Agr.,  Bur.  Chem.  Bui.  107  (rev.),  1908. 


45 

It  was  found  that  by  the  above  procodure  the  phosphomolybdate 
is  thrown  down  in  four  hours'  time,  leaving  a  clear  supernatant  liquid. 
In  cases  where  it  is  possible  it  is  best  to  let  the  precipitate  stand  over- 
night, although  in  all  cases  where  duplicate  determinations  were  made 
with  some  standing  overnight  and  others  four  hours  for  the  deposition 
of  the  yellow  precipitate,  closely  agreeing  results  were  obtained.  The 
addition  of  excessive  amounts  of  ammonium  nitrate  to  the  hydro- 
chloric acid  extract  does  not  offset  the  influence  of  the  chlorids.  It  was 
found  that  the  presence  of  chlorids  caused  an  excessive  precipitation 
of  molybdic  acid  also. 


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